Refactor NposSolver to be generic over the election algorithm

[gpestana]

Currently, the NposSolver::weight() trait is generic over a frame_election_provider_support::WeightInfo which returns the weights of running two flavors of phragmen.

This means that the NposSolver trait is not ergonomic for developer who'd like to use their own election algorithm. We could either:

i) refactor NposSolver to be more generic; or
ii) add a new trait ElectionSolver that is completely agnostic wrt to the election algorithm used.

[gpestana]

Related to paritytech/substrate#8250 and paritytech/substrate#12588

@kianenigma as discussed, this is probably not very important, but opening this issue for idea bookeeping.