[Chronik] Index txs; add /tx/:txid HTTP endpoint

Summary:
We add transactions to the DB as new blocks are coming in using TxWriter. Until we index the mempool, we set the time_first_seen to 0.

/tx/:txid returns txs by txid from the DB using the QueryTxs struct; not from the mempool yet though, which will be in a future diff.

We also add a few new protobuf messages: Tx, OutPoint, TxInput, TxOutput, BlockMetadata and Network.

Test Plan: ninja check-crates && ninja && ./test/functional/test_runner.py chronik_tx

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13467