rpc: Add EnsureArgsman helper

Summary:
This refactor doesn't change anything for the rpc layer, but it helps a bit with removing gArgs

This is a backport of core#23936

Depends on D15695

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15696