rpc: Add CRPCCommand constructor which takes RPCHelpMan

Summary:
This allows the constructor to ask the rpc manager for the name of the
rpc method or the rpc argument names instead of having it manually
passed in.

Note: added underscores to some constructors' arguments to prevent shadowing fields, like previously done in D5906

This is a backport of core#19386 [2/5]
https://github.com/bitcoin/bitcoin/pull/19386/commits/faaeb2b0b347b40ce456a951eec5e820587e5b02

Depends on D10006

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10007