feature: add onFullstoryDidStartSession lifecycle API

[RyanCommits]

FE-466

With the Guides & Services work, it revealed a requirement to tap into these native callback APIs: iOS & Android . The requirement is that on React Native, we should be able to react when a session splits or resumes. Right now we have an onReady API that accepts one-off promises that get resolved when a session starts.

Changes

1. Non-breaking changes to the onReady API. Our current onReady API accepts one promise. Multiple uses to the FullStory.onReady().then() API deletes the previous. We now store multiple onReady promises in an array to allow multiple FullStory.onReady().then() calls.
2. Added onFullstoryDidStartSession. Used the official React Native new architecture event emitter docs. This API is only supported on the new architecture, and will no-op on old architecture. This work is part of a push to get Guides & Surveys done, so I think legacy architecture support is optional for now.

This API maps to onReady and fullstoryDidStartSession on Android and iOS, respectively. If recordOnStart is true, it is likely that the session will already have started before onFullstoryDidStartSession is registered on the JavaScript side. Therefore will only respond to subsequent session starts. Users will have to continue to leverage the on onReady API to capture a session that is already in progress.

Tested on RN 66 & 84.

[RyanCommits]

We now set the delegate on app start, whereas before we set it when we call onReady