refactor: Replace JSONRPCRequest fHelp field with mode field

Summary:
No change in behavior

This is a backport of core#21035 [1/3]
https://github.com/bitcoin/bitcoin/pull/21035/commits/6158a6d3978a18d5ac4166ca2f59056d8ef71c3d

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10764