Expose "system" as a type.

[kye shimizu]

Chore

Describe the chore

Currently there is no type for "system", only the 3 main available ones that are here.
https://supabase.com/docs/guides/realtime

But for users using postgres changes, it is essential to be able to restart the connection.
This is also something discussed in #234 and #5641, but as the solution is heavy, this might be something to ease the pain for now.
https://github.com/GaryAustin1/Realtime2/blob/main/src/app.js#L57

https://github.com/orgs/supabase/discussions/5641

This chore might also extend to creating documentation for what "system" is.

Additional context

Add any other context or screenshots that help clarify the task.