PostgreSQL DLL available as Server Assembly in InMemory

We can of course connect to PostgreSQL in Data Discovery and there's also an InMemory data source available. Tick.

The InMemory data source, however, currently requires a local npgsql.dll file (which can be found in one of the Data Discovery folders). I suggest we make this DLL available as a Server Assembly such that the data source can be configured using credentials only (no mocking around with DLLs).