Make the RPCHelpMan aware of JSONRPCRequest and add Check() helper

Summary:
This is a partial backport of Core PR16240 : https://github.com/bitcoin/bitcoin/pull/16240/commits/c7a9fc234f3ce400ce78b9b434d2d210b2646c50

Depends on D6497

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6498