create a key exchange interface

[robdefeo]

Is your feature request related to a problem? Please describe.
With different cypto packages, secp256k1, ed25519, sr25519 with potentially more a consistent way to exchange keys will make symmetric encryption easier

Describe the solution you'd like
Create key exchange interface similar to

type KeyExchange interface {
	GenerateEphemeralKey() (private crypto.PrivateKey, public crypto.PublicKey, err error)
	ComputeSharedSecret(ephemeralKey crypto.PrivateKey, recipientKey crypto.PublicKey) ([]byte, error)
}