Anvil Tempo Builtin Fee Payer

Added a built-in Tempo fee payer to anvil --network tempo. eth_signRawTransaction sponsor-signs sender-signed Tempo AA transactions, and raw transactions carrying the sponsorship placeholder are sponsored automatically on submission, so cast send --sponsor-url and the Tempo SDK relay transport can point directly at anvil. The sponsor account defaults to the last dev account and is configurable via --tempo.fee-payer.