Improved typings (opensearch-project#3302)

* Improved typings

Passing this first param to `CoreSetup` generic would set a proper type to `depsStart` returned from `core.getStartServices()`

Signed-off-by: Aleksandar Djindjic <djindjic@gmail.com>

* Nit - rename example deps prop

Signed-off-by: Josh Romero <rmerqg@amazon.com>

---------

Signed-off-by: Aleksandar Djindjic <djindjic@gmail.com>
Signed-off-by: Josh Romero <rmerqg@amazon.com>
Co-authored-by: Josh Romero <rmerqg@amazon.com>
Co-authored-by: Miki <miki@amazon.com>
Signed-off-by: David Sinclair <david@sinclair.tech>

src/core/CONVENTIONS.md

@@ -171,7 +171,7 @@ import { MyAppRoot } from './components/app.ts';
  */
 export const renderApp = (
   core: CoreStart,
-  deps: MyPluginDepsStart,
+  deps: MyPluginStartDeps,
   { element, history }: AppMountParameters
 ) => {
   ReactDOM.render(<MyAppRoot core={core} deps={deps} routerHistory={history} />, element);
@@ -182,10 +182,10 @@ export const renderApp = (
 ```ts
 // my_plugin/public/plugin.ts
 
-import { Plugin } from '../../src/core/public';
+import { Plugin, CoreSetup } from '../../src/core/public';
 
 export class MyPlugin implements Plugin {
-  public setup(core) {
+  public setup(core: CoreSetup<MyPluginStartDeps>) {
     core.application.register({
       id: 'my-app',
       async mount(params) {
@@ -200,14 +200,14 @@ export class MyPlugin implements Plugin {
 }
 ```
 
-Prefer the pattern shown above, using `core.getStartServices()`, rather than store local references retrieved from `start`. 
+Prefer the pattern shown above, using `core.getStartServices()`, rather than store local references retrieved from `start`.
 
 **Bad:**
 ```ts
 export class MyPlugin implements Plugin {
  // Anti pattern
   private coreStart?: CoreStart;
-  private depsStart?: DepsStart;  
+  private depsStart?: DepsStart;
 
   public setup(core) {
     core.application.register({
@@ -218,7 +218,7 @@ export class MyPlugin implements Plugin {
         return renderApp(this.coreStart, this.depsStart, params);
       }
     });
-  }  
+  }
 
   public start(core, deps) {
     // Anti pattern
@@ -359,5 +359,5 @@ Migration example from the legacy format is available in `src/core/MIGRATION_EXA
 
 ### Naming conventions
 
-Export start and setup contracts as `MyPluginStart` and `MyPluginSetup`. 
+Export start and setup contracts as `MyPluginStart` and `MyPluginSetup`.
 This avoids naming clashes, if everyone exported them simply as `Start` and `Setup`.