feat(realtime): throw exception when try to update postgres change

[diegofesanto]

• after subscribe on postgres changes it should throw exception if try to change its configuration

What kind of change does this PR introduce?

feature

What is the current behavior?

What is the new behavior?

• throw a exception if try update config of postgres change after subscribe

Additional context

• related to parity(realtime): block setting postgres_changes listener after channel joins [from supabase-js] #242

[Tr00d]

Well done, thanks!
One note: I'll flag this as a breaking change though, cause we're introducing a new runtime behaviour. It was failing silently before, and now we're raising an exception.