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

substrate/Cargo.lock

@@ -6092,6 +6092,7 @@ dependencies = [
  "parking_lot 0.10.2",
  "pdqselect",
  "rand 0.7.3",
+ "rand_chacha 0.2.2",
  "sc-block-builder",
  "sc-client-api",
  "sc-consensus-epochs",
@@ -6425,6 +6426,7 @@ version = "2.0.0-rc3"
 dependencies = [
  "derive_more",
  "hex",
+ "merlin",
  "parking_lot 0.10.2",
  "rand 0.7.3",
  "serde_json",
@@ -7456,6 +7458,7 @@ dependencies = [
  "sp-application-crypto",
  "sp-consensus",
  "sp-consensus-vrf",
+ "sp-core",
  "sp-inherents",
  "sp-runtime",
  "sp-std",
@@ -7511,6 +7514,7 @@ dependencies = [
  "pretty_assertions",
  "primitive-types",
  "rand 0.7.3",
+ "rand_chacha 0.2.2",
  "regex",
  "schnorrkel",
  "serde",