pallet_authority_discovery: introduce current_authorities and next_authorities methods (#7892)

* split authorities discovery keys for the current and next session

* Revert "split authorities discovery keys for the current and next session"

This reverts commit 0a40b8b4c14e85d95357a27f6db30199cbe0aa4d.

* pallet_authority_discovery: introduce a next_authorities method

* address feedback

* amend the doccomments