add a getcurrencyinfo RPC command

Summary: This allows users to verify, after the node has already been started whether, the "-ecash" commad line parameter was specified.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9651