feat: new “extend” method for makeServicePlugin

## API

```
import { makeServicePlugin } from ‘feathers-vuex’
import { feathersClient } from ‘./feathers-client.js’

class Todo { /* truncated */ }

makeServicePlugin({
  Model: Todo,
  service: feathersClient.service(‘todos’),
  extend({ store, module }) {
    // Listen to other parts of the store
    store.watch(/* truncated */)

    return {
      state: {},
      getters: {},
      mutations: {},
      actions: {}
    }
  }
})

The old options for customizing the store are still fully functional.  The `module` object in the options contains the default state, getters, mutations, and actions merged with the ones provided in the options.  They are made available in case this is useful; however, they do not need to be returned  `extend` method’s return object.  The object returned from `extend` will be merged over the top of the ones in `module`.

As a result of this change, the following options are now deprecated in the `makeServicePlugin` method:

- state
- getters
- mutations
- actions

## History and Relevance

Up until now, `makeServicePlugin` and `makeAuthPlugin` have supported passing `state`, `getters`, `mutations`, and `actions` as keys in the options as shown here:

![Screen Shot 2020-10-29 at 1 14 15 PM](https://user-images.githubusercontent.com/128857/97621627-c8637380-19e8-11eb-9523-98a485440ef0.jpg)

This is nice and clean until you need access to the `store` variable to access APIs like `store.watch` (when you want a clean way to define related logic between multiple vuex modules).

This issue specifically showed up when I wanted a purely-logical, non-template-based method of watching for authentication from Auth0.  Up until now, I have always used the “Gateway Component” pattern that has a conditional in it for if auth is present, then put the majority of the app logic in a child component.  As shown in the above example, this extra logic can now live inside the same module as the rest of the service and vuex plugin definition.

src/service-module/make-service-module.ts

@@ -3,27 +3,37 @@ eslint
 @typescript-eslint/explicit-function-return-type: 0,
 @typescript-eslint/no-explicit-any: 0
 */
+import _pick from 'lodash/pick'
+import _merge from 'lodash/merge'
 import makeDefaultState from './service-module.state'
 import makeGetters from './service-module.getters'
 import makeMutations from './service-module.mutations'
 import makeActions from './service-module.actions'
 import { Service } from '@feathersjs/feathers'
 import { MakeServicePluginOptions } from './types'
+import { Store } from 'vuex'
 
 export default function makeServiceModule(
   service: Service<any>,
-  options: MakeServicePluginOptions
+  options: MakeServicePluginOptions,
+  store: Store<any>
 ) {
-  const defaultState = makeDefaultState(options)
-  const defaultGetters = makeGetters()
-  const defaultMutations = makeMutations()
-  const defaultActions = makeActions(service)
-
-  return {
+  const defaults = {
     namespaced: true,
-    state: Object.assign({}, defaultState, options.state),
-    getters: Object.assign({}, defaultGetters, options.getters),
-    mutations: Object.assign({}, defaultMutations, options.mutations),
-    actions: Object.assign({}, defaultActions, options.actions)
+    state: makeDefaultState(options),
+    getters: makeGetters(),
+    mutations: makeMutations(),
+    actions: makeActions(service)
   }
+  const fromOptions = _pick(options, [
+    'state',
+    'getters',
+    'mutations',
+    'actions'
+  ])
+  const merged = _merge({}, defaults, fromOptions)
+  const extended = options.extend({ store, module: merged })
+  const finalModule = _merge({}, merged, extended)
+
+  return finalModule
 }

src/service-module/make-service-plugin.ts

@@ -3,7 +3,12 @@ eslint
 @typescript-eslint/explicit-function-return-type: 0,
 @typescript-eslint/no-explicit-any: 0
 */
-import { FeathersVuexOptions, MakeServicePluginOptions } from './types'
+import {
+  FeathersVuexOptions,
+  MakeServicePluginOptions,
+  ServicePluginExtendOptions
+} from './types'
+
 import makeServiceModule from './make-service-module'
 import { globalModels, prepareAddModel } from './global-models'
 import enableServiceEvents from './service-module.events'
@@ -13,6 +18,14 @@ import _get from 'lodash/get'
 interface ServiceOptionsDefaults {
   servicePath: string
   namespace: string
+  extend: (
+    options: ServicePluginExtendOptions
+  ) => {
+    state: any
+    getters: any
+    mutations: any
+    actions: any
+  }
   state: {}
   getters: {}
   mutations: {}
@@ -25,6 +38,7 @@ interface ServiceOptionsDefaults {
 const defaults: ServiceOptionsDefaults = {
   namespace: '', // The namespace for the Vuex module. Will generally be derived from the service.path, service.name, when available. Otherwise, it must be provided here, explicitly.
   servicePath: '',
+  extend: ({ module }) => module, // for custom plugin (replaces state, getters, mutations, and actions)
   state: {}, // for custom state
   getters: {}, // for custom getters
   mutations: {}, // for custom mutations
@@ -91,7 +105,7 @@ export default function prepareMakeServicePlugin(
     return store => {
       // (1^) Create and register the Vuex module
       options.namespace = makeNamespace(namespace, servicePath, nameStyle)
-      const module = makeServiceModule(service, options)
+      const module = makeServiceModule(service, options, store)
       // Don't preserve state if reinitialized (prevents state pollution in SSR)
       store.registerModule(options.namespace, module, { preserveState: false })
 

src/service-module/service-module.state.ts

@@ -107,6 +107,7 @@ export default function makeDefaultState(options: MakeServicePluginOptions) {
     'instanceDefaults',
     'setupInstance',
     'handleEvents',
+    'extend',
     'state',
     'getters',
     'mutations',

src/service-module/types.ts

@@ -37,6 +37,11 @@ export interface HandleEvents {
   removed?: Function
 }
 
+export interface ServicePluginExtendOptions {
+  store: Store<any>
+  module: any
+}
+
 export interface MakeServicePluginOptions {
   Model: any
   service: Service<any>
@@ -64,6 +69,15 @@ export interface MakeServicePluginOptions {
   instanceDefaults?: () => {}
   setupInstance?: (data: any, { models, store }) => {}
   handleEvents?: HandleEvents
+
+  extend?: (
+    options: ServicePluginExtendOptions
+  ) => {
+    state?: any
+    getters?: any
+    mutations?: any
+    actions?: any
+  }
   state?: {}
   getters?: {}
   mutations?: {}