[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
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, majcosta
Reviewed By: #bitcoin_abc, majcosta
Differential Revision: https://reviews.bitcoinabc.org/D6503