Register a signer key for an account. Requires two EIP-712 signatures:

1. Account signs RegisterSigner:

RegisterSigner(address signer, string message, uint40 expiration, uint256 nonce)

Struct hash: keccak256(abi.encode(TYPEHASH, signer, keccak256(bytes(message)), expiration, nonce))

2. Signer signs VerifySigner:

VerifySigner(address account, uint256 nonce)

Struct hash: keccak256(abi.encode(TYPEHASH, account, nonce))

Both use the same EIP-712 domain from GET /v1/auth/eip712-domain.