Sends a user operation with a signature. The payload to sign must be the userOpHash field of the user operation. This hash should be signed directly (not using personal_sign or EIP-191 message hashing). The signature must be 65 bytes in length, consisting of: - 32 bytes for the r value - 32 bytes for the s value - 1 byte for the v value (must be 27 or 28) If using the CDP Paymaster, the user operation must be signed and sent within 2 minutes of being prepared.