Support chaining the mapper configuration

[herebebeasties]

I would like to go:

new NpgsqlDataSourceBuilder(connectionString)
            .UseNodaTime()               // <-- currently returns INpgsqlTypeMapper
            .UseLoggerFactory(logging)   // <-- which doesn't work for subsequent chained calls
            .Build();

UseNodaTime() returns the interface INpgsqlTypeMapper and not the NpgsqlDataSourceBuilder, which breaks the fluent API here. This PR uses a generic type constraint to fix this.

[herebebeasties]

@roji any chance of landing this one at some point, given it's a one-liner?

[herebebeasties]

@roji ?

[roji]

I unfortunately don't have any time to look at PRs at the moment, due to intensive work on the .NET/EF side. Things should be better in September.

[NinoFloris]

This was implemented in #5918 across all plugins