Add WhoisResponse class

[irl]

The client-server spec talks about an endpoint at /_matrix/client/r0/admin/whois/{userId} that can be used to look up information about a user, but the response type is not implemented in matrix-nio.

I'm using this in another project and would be happy to take a go at implementing this. Let me know if someone is already working on it.

There are sub-schemas "DeviceInfo", "SessionInfo" and "ConnectionInfo". I'm not sure if each would be their own class or if it's preferred to just parse it all into one object.