[Chronik] Enable indexing txs by plugins + return protobuf

Summary:
Make Chronik run plugins on txs on block connect (using PluginsWriter), add PluginEntry to the protobuf and return indexed plugin outputs for each tx if available.

We currently don't index mempool txs, which will be done in a future diff.

Depends on D16535.

Test Plan: ./test/functional/test_runner.py chronik_plugins chronik_plugins_setup

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D16537