[terraform] add GSI for farcasterID

Summary:
We need this index to query efficiently for the userID, username, and walletAddress for all users with a farcasterID

We set the projection_type to INCLUDE so we don't just get the userID

Depends on D11399

Test Plan: later in the stack I introduce a database client method to get the

Reviewers: bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11400