Enable BEEFY Keystore to produce BLS signature

[drskalman]

Currently happening at https://github.com/drskalman/substrate/blob/skalman-bls-beefy/client/beefy/src/keystore.rs#L77

It only support signing ecdsa signature. However, in the case, where beefy message is also going to contain a single aggregated signature, this need to be change. The easiest path to this is to have AggregatableBeefyKeyStore trait which will generate and aggregate BLS signature. One could implemenet this trait for MMRed BEEFY signature and do nothing in this way the beefyclient code does not need to be changed wether we are dealing with MMRed Aggregated or BLS aggregated signature.

[drskalman]

231a38e610 implements BeefyBLSKeystore and BeefyBLSnECDSAKeystore. Still needs to add tests and adapt BEEFY client's worker.rs to use the new structs.

[drskalman]

ea40ab92ae13e89388fee76122c99f3b382763e8:

• resolve errors in ECDSAnBLSKeystore implementation.
• Make beefy primitive testing compile again.
• Add bls_sign to local keystore.