Return the blockhash of the tx mining block in gettransactionstatus

Summary: This is only available if the transaction indexer is enabled.

Test Plan:

./test/functional/test_runner.py rpc_gettransactionstatus

Reviewers: #bitcoin_abc, tobias_ruck

Reviewed By: #bitcoin_abc, tobias_ruck

Differential Revision: https://reviews.bitcoinabc.org/D16658