Add an extra_constant to pallet-collator-selection (#7206)

Currently `pallet-collator-selection` does not expose a way to query the
assigned pot account derived from the `PotId` configuration item.
Without it, it is not possible to transfer the existential deposit to
it.

This PR addresses this issue by exposing an extra constant.

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Co-authored-by: Bastian Köcher <git@kchr.de>