Implementing `MaxEncodedLen` for `generate_solution_type` (#11032)

* Move `sp-npos-elections-solution-type`
to `frame-election-provider-support`
First stab at it, will need to amend some more stuff

* Fixing tests

* Fixing tests

* Fixing cargo.toml for std configuration

* Implementing `MaxEncodedLen`
on `generate_solution_type`

* Full implementation of `max_encoded_len`

* Fixing implementation bug
adding some comments and documentation

* fmt

* Committing suggested changes
renaming, and re exporting macro.

* Removing unneeded imports

* Implementing `MaxEncodedLen`
on `generate_solution_type`

* Full implementation of `max_encoded_len`

* Fixing implementation bug
adding some comments and documentation

* Move `NposSolution` to frame

* Implementing `MaxEncodedLen`
on `generate_solution_type`

* Full implementation of `max_encoded_len`

* Fixing implementation bug
adding some comments and documentation

* Fixing test

* Removing unneeded dependencies

* `VoterSnapshotPerBlock` -> `MaxElectingVoters`

* rename `SizeBound` to `MaxVoters`

* Removing TODO and change bound

* renaming `size_bound` to `max_voters`

* Enabling tests for `solution-type`
These got dropped off after the crate was moved from `sp_npos_elections`

* Adding tests for `MaxEncodedLen` of solution_type

* Better rustdocs. Better indenting and comments.
Removing unneeded imports in tests.