Allow for 'partially constructed' generics for UseDefaultDataSource/Behavior, or throw an exception when the arity of the implementation type doesn't match the service type.

[ascott18]

For example, it doesn't work currently to create class MyDataSource<T> : StandardDataSource<T,AppDbContext> and register it as a default data source.

Either find a way to make this work, or throw an exception when the arity of the implementation type doesn't align.