[proto][identity] add optional fid to registration request messages

Summary:

• update proto
• add farcaster id to dynamodb users table on registration
• update rust clients to send None for now (will send actual FID in next diff)
• update grpc-web bindings with farcaster id in registration requests

Test Plan: tested in subsequent diff by actually calling the updated register RPC from native

Reviewers: bartek, ashoat

Reviewed By: bartek, ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11396