Firestore Library Upgrade

Main PR for all the Firestore v2 upgrade.

Part 1 [✅]

PR: #7298

This PR contains all the additive changes to the library laying the ground work for the further work.

• Add gapic client configs.
• Add credentials wrapper support.
• Add root serializer and request handler.
• Update resource classes to add serializer and request handler to:
  • Constructors
  • Instantiations
• Add serializer and request handler support in all the snippet and unit tests.
• Update core's version to a future non existent version

Part 2 [✅]

PR: #7300

Update the 9 RPCs exposed by Grpc connection class and correspondingly update their unit and system tests.

Part 3 [✅]

PR: #7418

Do the rest of the work like:

• Update the system tests if something breaks
• Add MIGRATION.md file
• Update the documentation whereever required.
  • Snippets
  • Doc blocks
  • FirestoreClient constructor's documentation
• Remove old GAPIC files
  • Class aliases
  • Gapic client
• Update core's version to the next future version.
• Mark classes as @internal where ever required.

BREAKING_CHANGE_REASON=RC-1 for the version 2 of the Firestore Library