Add EVENT TRIGGER for schema cache auto reload

[steve-chavez]

We need an EVENT TRIGGER for NOTIFYing our db channel and reloading the schema cache.

There's already some progress on #1512 (comment).

I was thinking this would be a pure doc issue(PostgREST/postgrest-docs#335) but now I think it's a core issue that could use some testing(bash test) on the main repo.

[steve-chavez]

Already solved with docs: https://postgrest.org/en/latest/schema_cache.html#finer-grained-event-trigger