[Code health] Front-load anticipated breaking changes before Beta release

[gino-m]

This includes:

• [Code health] Make representation of user data (task values) more extensible #1703
  • [Code health] Support new structured submission data representation ground-android#2431
• [Code health] Use field numbers instead of names as keys in Firestore #1761
• [Code health] Rename remaining instances of field to task #936
• [Code health] Clean up remaining references to old responses field #1288
• Split survey document into multiple documents and collections #1536

@jcqli FYI

@rfontanarosa @shobhitagarwal1612 @sufyanAbbasi FYI

[jcqli]

Synced with @gino-m to clean up some of this task tracking:

• The top half tasks under "This includes:" will be addressed by the protobuf migration. Let's leave them open as outstanding issues but close them out via the migration tasks
• We'll move the migration tasks under "Migration plan:" to be tracked by [PROJECT] New proto-backed Firestore structure #1758 as the parent issue. It will track issues across both repos so that we can execute them in order as needed.

I'll make these changes to cleanup our issue tracking

[gino-m]

All related work on this issue is being handled as part of #1758.