Refinements to VRF types (#14036)

* Allow extra signing data

* Fix tests after renaming

* Rename VrfSecret/VrfVerifier to VrfSecret/VrfPublic

* Further encrapsulation of 'transcript' type to the sr25519 implementation

* Keystore sr25519 pre-output

* Leave additional custom input field hidden in the associated VrfInput type

* Fix test

* More ergonomic output_bytes

* Trigger pipeline

* Define a separated type for vrf signature data

* Fix docs

* Fix doc

* Remove annotation

* Directly use dleq_proove and dleq_verify in sr25519

* Trigger CI

* Remove cruft before merge