pgtype: enable deep copy of pgtype.Map

I would like to design a "clone" connection function in my program, which takes a parent connection, and run `pgx.ConnectConfig()` with the same config of the parent connection, and also deep copy the type map of the parent connection.

However, I can't find a way to do such a deep copy, as none of `Map.oidToType` or `nameToType` and etc are exported.

So I wonder if we can implement a method like `func (m *Map) Clone() Map`, or just simply make these not-exported field accessible so that i can loop over entries inside each map.

Start at the pgtype.Map entry point and inspect its unexported type maps and related state. Decide whether Clone should return an independent deep copy or expose internals, then add coverage demonstrating that changes to the clone do not affect the original.

Written by the indexing model from the issue text.