Add clientbound handshake response packet

[carl-anders]

I'm unsure if this is the way we want to implement this. The response is unlike any other packet, so it needs some special handling. We're kinda "lucky" that "\x03Hey OVR =D 5" fits nicely as:

• u32 id (3"Hey")
• u64 sequence (" OVR =D ")
• u8 for the version ("5")

Doing it this way works but feels very hacky?

[TheDevMinerTV]

Protip: don't handle the packet content at all, just like the actual firmware :D

[TheButlah]

Check clippy