Expose ValidatedTransaction from transaction pool (#11588)

This is required to make a tx pool wrapper for some custom transaction validation, specificially
required to make a wrapper around the `LocalTransactionPool` implementation (https://github.com/subspace/subspace/blob/f54881a9b5/crates/subspace-service/src/pool.rs#L232-L269).

Related: https://github.com/paritytech/substrate/discussions/11520