New sub-trait to mock staking miner (#11350)

* new separate config trait for staking miner

* fix some docs and stuff

* relax trait bounds

* some cleanup

* Update frame/election-provider-multi-phase/src/unsigned.rs

* add comment

* self review and fix build

* fix docs

Co-authored-by: Niklas Adolfsson <niklasadolfsson1@gmail.com>