babe: make secondary slot randomness available on-chain (#7053)

* babe: make secondary slot randomness available on-chain

* babe: extract out vrf_output function

* babe: add missing comment

* babe: fix incorrectly storing primary randomness

* babe: add test for onchain author vrf

* babe: fix reviewer nits

* runtime: bump spec_version

* babe: remove outer Option for AuthorVrfRandomness

* babe: fix reviewer nits on doc strings

* babe: move make_vrf_output to mock.rs

* babe: cleanup docs

* babe: kill ephemeral entry instead of take

* babe: use type alias for maybe randomness

Co-authored-by: André Silva <[email protected]>