Implement AuthClient.respond

[bkrem]

Steps

1. B constructs message to be signed from request and signs it. - Optional for now, i.e. we can just send a mock string like "signature" back at first. We still need to figure out how to best do the EIP-191 signing (most likely ethers.js)
2. B generates their own keyPair Y - i.e. const responderPubKey = await this.client.core.crypto.generateKeyPair()
3. B uses sendResult for auth_request on response topic with the TYPE_1 option, which generates shared symKey R via crypto.encode and encrypts the message with it - Example use of TYPE_1 here

Goals

• Responder can receive incoming auth request
• Responder can send a type 1 envelope response containing a (mock) signature