Added Execute(JSONRPCRequest) overload in RPCCommandBase for wallet and help commands

Summary:
This is necessary for migration of wallet and help RPC commands that depend on
the entire JSONRPCRequest context.

Test Plan: ninja check

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2616