BEEFY: Define a `BeefyVerify` trait for signatures (#12299)

* Define CustomVerify trait

Signed-off-by: Serban Iorga <serban@parity.io>

* Use ECDSA CustomVerify for MultiSignature

Signed-off-by: Serban Iorga <serban@parity.io>

* beefy: small simplifications

Signed-off-by: Serban Iorga <serban@parity.io>

* Revert "Use ECDSA CustomVerify for MultiSignature"

This reverts commit 136cff82505662dd92c864491814629d2bc349f0.

* Revert "Define CustomVerify trait"

This reverts commit adf91e9e6d1bdea6f00831f6067b74c3d945f9a2.

* Define BeefyAuthorityId and BeefyVerify traits

* Improve BeefyVerify unit tests

Co-authored-by: Robert Hambrock <roberthambrock@gmail.com>

* fmt & import sp_core::blake2_256

* Renamings

* remove SignerToAccountId

* fix

Signed-off-by: Serban Iorga <serban@parity.io>
Co-authored-by: Robert Hambrock <roberthambrock@gmail.com>