WIP: feature-gated single block/multi block miner

[niklasad1]

Continuation of #910

[kianenigma]

Please update this branch to work based on paritytech/polkadot-sdk#7282.

From this pallet, you can pull:

1. pallet-election-provider-multi-phase, which remains as is and should be the basis for the single page election. The single page election should not really change.
2. pallet-election-provider-multi-phase, which should have the multi-block miner in it.

The overall game has changed a bit:

• You listen to Phase::Signed(_) to be opened.
• Then, this struct is your new miner: https://github.com/paritytech/polkadot-sdk/pull/7282/files#diff-7bf4ce888f36b99a5dcf6afec11a65d952883bbbc7a3353ac4934fab10f5a938R123-R125
• You need to first fetch "how many pages should I mine"? This is in epmb::Confgi::Pages and in metadata.
• Call mine_solution(pages, true).
• This will give you: PagedRawSolution: https://github.com/paritytech/polkadot-sdk/pull/7282/files#diff-8f1b0ab150c06e68b6ac2c1d4a1954c96558ee54ceb34e551cf774c7b979c434R71
• From here, first submit register(round, score)
• Then submit all subsequent pages via submit_page(_)
• High level, this stuff is also documented in ./src/signed/mod.rs