Support multiple Firebase instances

[DavidM-D]

Right now we select a users key using the pure(ish) function ID Token => Key. We need to allow multiple Firebase instances, but we don't want one Firebase instance to be able to hijack the keys of another instance.

Therefore we need to change the key selection function to (Firebase ID, ID Token) => Key, while preserving the keys of existing near.org users.

We also need to be able to have, on the leader node, a configurable list of accepted Firebase IDs.

[volovyks]

Planned design

• YAML/TOML config in the repo (empty)
• Values in Secret Manager
• Vector of partners (aud, relayer_url, api_key) in that config, configurable in GCP
• We can get partner from OIDC Id Token