Add a dumpcoins RPC to dump the coins available in the wallet

Summary: As per title. This turned useful for debugging, so I productionized it.

Test Plan:

ninja all check-all

Added a simple integration test.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8670