FIP: Onchain Signers

Farcaster's proposed FIP would track app signers onchain in a Key Registry on an L2 contract instead of in the current offchain CRDT, to avoid usability problems. The proposal maps keys to fids rather than custody addresses, so the signer relationship persists when an fid changes custody, and onchain storage avoids the CRDT's signer limit, where adding a signer past the cap removes the earliest one. Under the proposal, the Key Registry is deployed to the L2 at a stated address, and after a migration timestamp is set, hubs will reject offchain signers created at or after that timestamp.