[keyserver][lib] Add Neynar config for keyserver/web/landing

Summary:
It isn't needed on landing yet, but it gets included automatically so I figured that there's no downside.

For keyserver it will be accessed via getCommConfig. On web and landing it will be access via process.env.COMM_NEYNAR_KEY.

It will be configured either by setting keyserver/secrets/neynar.json, or by the env var COMM_JSONCONFIG_secrets_neynar.

This approach follows what we're done for the Alchemy and WalletConnect API keys.

This addresses ENG-7424.

Test Plan:
I tested this in combination with later diffs on both keyserver and web:

1. I specified keyservers/secrets/neynar.json
2. For keyserver, I tested that Farcaster usernames worked in notifs
3. For web, I tested that Farcaster usernames were resolved in robotext

Reviewers: ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11874