Propose parachain pallet for Rococo (#1607)

* Adds a new pallet for proposing parachains on rococo

* Finish first version

* Remove the validators after deregistering the parachain

* Docs and other fixes

* Enable it in the runtime

* Be more smart ;)

* Store validity_guarantors in a vec instead of a hash set

If multiple parties don't have set their session keys, this code will
otherwise merge both of these validators into "one validator". This
leads to PovBlock being sent to the runtime without the required number
of votes and that makes the block building fail, because the runtime use
a slightly different algorithm to calculate the number of validity votes.

* Implement the correct origin for the pallet in the runtime

* Update runtime/rococo/src/lib.rs

Co-authored-by: Dan Forbes <[email protected]>

* Fix runtime benchmarks

Co-authored-by: Dan Forbes <[email protected]>