Add ability to configure NpgsqlDataSource

[jack775544]

Description

When using the AddNpgsqlDbContext or AddNpgsqlDataSource extension methods, there is no way to configure the NpgsqlDataSource that is registered in the service collection.

Configuring the data source is important for things like enum type mapping and spatial support among other things.

Proposed Solution

Add another configure option onto the methods to allow for modification of the data source.

[eerhardt]

This should be fixed when #1403 is done.

[eerhardt]

This is now complete with #2125.