fix(gateway): make loadServiceDefinitions a private method, replace with reasonable API

[trevor-scheer]

Right now, there's a use case for subclassing ApolloGateway in order to override the loadServiceDefinitions method. This is very effectively similar to providing a function to the experimental_updateServiceDefinitions "hook", however said hook doesn't have access to the original loadServiceDefinitions fn on the gateway which can be useful.

A few reasonable alternatives to this (proposed by @glasser):

• pass a reference to loadServiceDefinitions into the experimental_updateServiceDefinitions hook
• create a willUpdateServiceDefinitions hook which can modify the result of the gateway's standard service defs update
• some other hook/config option which is more explicitly dedicated to service overrides

ref: #539

[gtkatakura]

I am using exactly this case you said about using the original loadServiceDefinitions today to support the relay spec into the gateway:

  protected async loadServiceDefinitions(config: GatewayConfig) {
    const defs = await super.loadServiceDefinitions(config)

    if (!defs.isNewSchema || !defs.serviceDefinitions) return defs

    await this.mergeLocalServiceDefinitions(defs.serviceDefinitions)
    await this.mergeLocalGraphQLModule(defs.serviceDefinitions)