Collation protocol: stricter validators (#2810)

* guide: declare one para as a collator

* add ParaId to Declare messages and clean up

* fix build

* fix the testerinos

* begin adding keystore to collator-protocol

* remove request_x_ctx

* add core_for_group

* add bump_rotation

* add some more helpers to subsystem-util

* change signing_key API to take ref

* determine current and next para assignments

* disconnect collators who are not on current or next para

* add collator peer count metric

* notes for later

* some fixes

* add data & keystore to test state

* add a test utility for answering runtime API requests

* fix existing collator tests

* add new tests

* remove sc_keystore

* update cargo lock

Co-authored-by: Andronik Ordian <[email protected]>