[keyserver][lib][native][web] Add support for resolving FC usernames to getEntityTextAsString
Summary:
getEntityTextAsString is used in two places:
- Notifs, where we potentially need to resolve robotext with FC users in it
- Determining the name of a pending thread. If somebody is creating a pending thread from a robotext that has FC users in it, we want to resolve the FC usernames
Depends on D11880
Test Plan: I tested notifs generated on the keyserver and delivered to an Android device, and confirmed that they resolved Farcaster usernames properly
Reviewers: ginsu
Reviewed By: ginsu
Subscribers: tomek
Differential Revision: https://phab.comm.dev/D11881