feat(analytics-core): remote config client

[Mercy811]

Summary

AMP-124564

Remote config client

• Have the same design and APIs as the iOS one https://github.com/amplitude/AmplitudeCore-Swift/pull/2
• A remote config client based on request-on-demand and subscription + callback modes.
• The remote config client has 3 public APIs. See code comment for more details.
• subscribe() is very similar to addEventListener(). Callback is called when the remote config fetched.
• Analytics SDK will initialize a remote config client and plugins can use it for request remote config on their needs.

export interface IRemoteConfigClient {
  subscribe(key: string | undefined, deliveryMode: DeliveryMode, callback: RemoteConfigCallback): string;

  unsubscribe(id: string): boolean;

  updateConfigs(): void;
}

Remote config storage

• Default to local storage. Ideally we can take advantages of browser storage instead of using local storage, but it requires extra response headers (AMP-126646) and work.

Checklist

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