Implement a spockets `RotTransport` for interacting with the Real SP

[andrewjstone]

There's an ongoing plan to have a specific RPC like mechanism baked into helios that allows us to send messages to the SP over the UART and receive responses. Helios will mediate access to the SP which will enable sprockets to talk to the RoT via the SP. Communicating with RoT is abstracted in the RotManager. To use the new helios mechanism, we will need to implement an RotTransport for it.

[andrewjstone]

This RPC mechanism is IPCC built by @citrus-it and @jgallagher . Requests reaching the host-sp-comms task in hubris will then be sent over sprot to the RoT.

[andrewjstone]

No longer a thing.