[chronik] implement server.version electrum method

Summary:
See https://electrum-cash-protocol.readthedocs.io/en/latest/

We simply return protocol version "1.4" at the moment, which is what fulcrum also report to Electrum ABC's current request.
This is not exact as we only implement a subset of the commands in that version of the spec, but we do follow the rules for the implemented methods (for instance we don't support some 1.5 additional parameters to blockchain.scripthash.history)

Test Plan:
ninja check-functional

Compare results and errors to what a Fulcrum instance returns for similar requests

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17676