[Go] Add retriever plugin for firestore vector store

[pavelgj]

Looks like it was added in the Firestore Go SDK
https://github.com/googleapis/google-cloud-go/releases/tag/firestore%2Fv1.16.0

[pavelgj]

Not 100% sure whether firestore vector store retriever should be part of firebase plugin (doesn't currently exist)... or google cloud plugin... or a standalone plugin. Perhaps worth a discussion on discord.

[cabljac]

#1079

rebased that PR on main and added docs, sample.

[cabljac]

#1079 PR into main