Decouple the session validators from im-online (#7127)

* Decouple the session validators from im-online

* .

* Add SessionInterface trait in im-online

Add ValidatorId in im-online Trait

Make im-online compile

Make substrate binary compile

* Fix merging issue

* Make all compile

* Fix tests

* Avoid using frame dep in primitives via pallet-session-common

* Merge ValidatorSet into SessionInterface trait

Wrap a few too long lines

Add some docs

* Move pallet-sesion-common into pallet-session

* Move SessionInterface to sp-session and impl it in session pallet

Ref https://github.com/paritytech/substrate/pull/7127#discussion_r494892472

* Split put historical::FullValidatorIdentification trait

* Fix line width

* Fix staking mock

* Fix session doc test

* Simplify <T as ValidatorIdentification<AccountId>>::ValidatorId as ValidatorId<T>

* Nits

* Clean up.

* Make it compile by commenting out report_offence_im_online bench

* Tests

* Nits

* Move OneSessionHandler to sp-session

* Fix tests

* Add some docs

* .

* Fix typo

* Rename to ValidatorSet::session_index()

* Add some more docs

* .

* Remove extra empty line

* Fix line width check

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* Apply suggestions from code review

* Cleaup Cargo.toml

* Aura has migrated to Pallet now

Co-authored-by: Tomasz Drwięga <tomasz@parity.io>