[identity] link farcaster account implementation

Summary:
use ddb update-item to add a farcaster ID to a user item. later in the stack, we add a conditon to avoid overwriting an existing farcasterID

Depends on D11405

Test Plan: RPC is called from native and web later in the stack

Reviewers: bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11406