[Chronik] Prefix WS protobuf enum entries with BLK_ or TX_

Summary: In protobuf, all entries in an enum share the same namespace, which is why we prefix them according to their type.

Test Plan: ninja && ./test/functional/test_runner.py chronik_ws.py && ./test/functional/test_runner.py chronik_ws_script.py

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13794