implement collator-selection for Encointer

[brenzi]

Encointer still operates with hard-coded collator keys which is very rigid and hinders decentralization of infrastructure.
This should finally be changed and aligned with other system parachains

There may be synergy when implementing this before #176

draft strategy:

1. instantiate the necessary pallets for release 1.3.0 but don't change collation logic yet (as the pallet is initialized with zero invulnerables)
2. resolve remove treasury from Encointer runtime #176 and replace Treasury with ToAuthor as fee beneficiary
3. deploy runtime upgrade 1.3.0
4. establish invulnerables = existing hard-coded keys
5. activate collator-selection, deprecate hard-coded collator keys

Questions:

• we may be able to use state migration to initialize the set of invulnerables and do all this in a single upgrade. Check how Asset Hub did it