Council can manage collator set

[brenzi]

We have an ecosystem initiative ongoing to cover operation costs for running collators by a diverse set of operators:

https://forum.polkadot.network/t/economic-model-for-system-para-collators/1010/25

It is in our interest to onboard new collators, but currently, only root could do that.

We could allow Encointer council to pick and drop individual collators. Depending on how above discussion progresses, there may also be a standard solution for system parachains that we can follow. In the best case, the choice of collators is out of our hands, but not sure how this could look like

We may need our own pallet for this because statemine does this differently. (TBV)