finder.PublicKeyFromAddress should return PublicKey

[robdefeo]

Is your feature request related to a problem? Please describe. The key of key is needed to return what encryption methods are allowed

Describe the solution you'd like Extend public key finder to return PublicKey

• [ ] In internal/mailbox/pubkey.go change PublicKeyFromAddress(ctx context.Context, protocol, network string, address []byte) ([]byte, error) to PublicKeyFromAddress(ctx context.Context, protocol, network string, address []byte) (cypto.PublicKey, error)
• [ ] use "github.com/mailchain/mailchain/crypto" package
• [ ] change internal/clients/etherscan/pubkey-finder.go to new signature. Use secpk256k1 package for PublicKey
• [ ] change internal/protocols/substrate/pubkey.go to new signature. Use ed25519 package for PublicKey
• [ ] run make generate to update mock
• [ ] update tests

Additional context Substrate will be extended in the future to support different key types

[epishovska]

@robdefeo Can I have this assigned to me?