rpc: migrate JSONRPCRequest functionality into request.cpp

Summary:
This si a partial backport of Core PR16240 : https://github.com/bitcoin/bitcoin/pull/16240/commits/5c5e32bbe3dfa790dd8bb421fbd6301ae10b09f5

Depends on D6496

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6497