Get transaction by hash

Look up a transaction by its hash. This is the same hash that is returned
by the API when submitting a transaction (see PendingTransaction).

When given a transaction hash, the server first looks for the transaction
in storage (on-chain, committed). If no on-chain transaction is found, it
looks the transaction up by hash in the mempool (pending, not yet committed).

To create a transaction hash by yourself, do the following:

1. Hash message bytes: "RawTransaction" bytes + BCS bytes of Transaction.
2. Apply hash algorithm SHA3-256 to the hash message bytes.
3. Hex-encode the hash bytes with 0x prefix.