Make internal constructors public

I want to create a derived class from the `SqlDataReader` but unfortunately its constructor is marked as `internal`.
```csharp
internal SqlDataReader(SqlCommand command, CommandBehavior behavior)
```
Making it `public` can be useful for mocking, overriding some of its methods, without using reflection, proxy libraries and so on. It would be great if this library uses the same approach as the EF-Core team is doing, by not defining `internal` types.