integrate dispute finality (#3484)

* finality_target adjustments

* fn finality_target

* partially address review comments

* fixins

* more rustic if condition

* fix tests

* fixins

* Update node/core/approval-voting/src/lib.rs

Co-authored-by: Andronik Ordian <[email protected]>

* Update node/core/approval-voting/src/lib.rs

Co-authored-by: Robert Habermeier <[email protected]>

* review comments part one

* rename candidates -> block_descriptions

* testing outline (incomplete, WIP)

* test foo

* split RelayChainSelection into RelayChainSelection{,WithFallback}, introduce HeaderProvider{,Provider}

* make some stuff public (revert this soon™)

* some test improvements

* slips of pens

* test fixins

* add another trait abstraction

* pending edge case tests + warnings fixes

* more test cases

* fin

* chore fmt

* fix cargo.lock

* Undo obsolete changes

* // comments

* make mod pub(crate)

* fix

* minimize static bounds

* resolve number() as before

* fmt

* post merge fix

* address some nits

Co-authored-by: Andronik Ordian <[email protected]>
Co-authored-by: Robert Habermeier <[email protected]>