modify `client/beefy/src/worker.rs` to use either ECDSA or BLSnECDSA keystores

[drskalman]

worker.rs currently only using BeefyKeystore to carry out crypto tasks, however in the new structure BeefyKeystore is only a trait now and the worker either need to instiate BeefyECDSAKeystor or BeefyBLSnECDSAKeystore depending if they want to certify their BEEFY messages using Merkle tree of ECDSA signature or an aggregated BLS signature.