refactor: Add RPC server ExecuteCommands function

Summary:
No change in behavior. New function is split from CRPCTable::execute and used in the next commit.

This is a backport of core#21035 [2/3]
https://github.com/bitcoin/bitcoin/pull/21035/commits/14f3d9b908ed9e78997bfaad3d8a06357a89d46e

Depends on D10764

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10765