Add key and signature types into Signature message

[realloc]

NeoFS uses different key and signature types, but this knowledge is not reflected directly in protocol definitions, only in code.

Signature structure needs to have key type reference and signature algorithm reference.

[realloc]

As an alternative approach, we could consider using multihash format for storing signature. However, if we add a Signature hash type field, functionally it would be almost the same thing. Need to discuss what's better in the long run.

[fyrchik]

I see 2 problems with multihash:

1. The codec in header describes only the type of data, not the signature algorithm https://github.com/multiformats/multicodec/blob/master/table.csv. So sha512 can mean both ECDSA and EdDSA and we still need another field).
2. It can be hard to add new algorithms there (for local regulations or custom requirements) without effectively forking the whole multicodec or multihash repo.

[cthulhu-rider]

Also we have some need to work with WalletConnect (c) @realloc