Provide method to retrieve parameterStatuses keys from

`PgConn` struct has a private `parameterStatuses` map field. The `PgConn.ParameterStatus` method returns the value of a specified key from that map. As far as I can tell, there is no mechanism to retrieve the set of keys in the map.

`PgConn` should expose either a clone of the `parameterStatuses` field (generated via `maps.Clone` maybe?) or an array of
the `parameterStatuses` keys that the caller can then use to call the existing `ParameterStatus` with.

Locate the PgConn struct, its private parameterStatuses map, and the existing ParameterStatus method. Review nearby connection API patterns before choosing how callers should enumerate parameter-status keys; the work is done when the public API exposes the requested information and its behavior is covered by tests.

Written by the indexing model from the issue text.