Add assignment keys to session keys, no separate approvals key (#2092)

* guide: merge backing and approval keys

* bump substrate master & update primitives

* use new SessionInfo struct in session_info

* session keys upgrade for Polkadot

* kusama & westend runtimes

* bump westend, kusama, and polkadot versions

* add session key to rococo & test-runtime

* update prepare-test-net to latest subkey

* update chain specs to support new para_assignment session key

* get cargo.lock from master

* formatting

* update kill_storage based on substrate master

* fix test-service

* assgn -> asgn

* use session info module for assignment session key