[rpc] expose ability to mock scheduler via the rpc

Summary:
This is a partial abckport of Core PR18037 : https://github.com/bitcoin/bitcoin/pull/18037/commits/8bca30ea17cd4c1dacee28eaa27e5fa3493b021d

Depends on D6502 and D6498

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6503