In #3032, we've introduced a callback that gets invoked when a physical connection is established, to allow the setting of settings and resources which need to be used across the lifetime of the connection (e.g. temporary tables). However, the callback receives an NpgsqlConnector, which is a pubternal type - it is currently only exposed publicly for plugins. It should not be present on our "real" public surface.

As discussed offline, we should decide what to do with this:

1. We could move the callback to NpgsqlConnection.Open, but with current factoring that means adding a check in the hot, pooled path, which doesn't seem like a good idea.
2. Passing an NpgsqlConnection at the early stage where it's at would introduce connector/connection binding issues we've tried to avoid. There may be a way to do this without bringing back all the mess.

/cc @vonzshik @NinoFloris