[native] add linkFarcasterAccount to native IdentityServiceContextProvider

Summary:
JS client method which can now be called when an existing Comm user wants to link their Farcaster account.

Depends on D11408

Test Plan: called the client method with an existing userID and confirmed in DDB that the user item now contained a FarcasterID

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11409