Babe VRF Signing in keystore (#6225)

* Introduce trait

* Implement VRFSigner in keystore

* Use vrf_sign from keystore

* Convert output to VRFInOut

* Simplify conversion

* vrf_sign secondary slot using keystore

* Fix RPC call to claim_slot

* Use Public instead of Pair

* Check primary threshold in signer

* Fix interface to return error

* Move vrf_sign to BareCryptoStore

* Fix authorship_works test

* Fix BABE logic leaks

* Acquire a read lock once

* Also fix RPC acquiring the read lock once

* Implement a generic way to construct VRF Transcript

* Use make_transcript_data to call sr25519_vrf_sign

* Make sure VRFTranscriptData is serializable

* Cleanup

* Move VRF to it's own module

* Implement & test VRF signing in testing module

* Remove leftover

* Fix feature requirements

* Revert removing vec macro

* Drop keystore pointer to prevent deadlock

* Nitpicks

* Add test to make sure make_transcript works

* Fix mismatch in VRF transcript

* Add a test to verify transcripts match in babe

* Return VRFOutput and VRFProof from keystore