Add tests for GetFieldValue checking possibility of getting a stream or text reader

[YohDeadfall]

Originally posted by @roji in npgsql/npgsql#2446 (comment):

As mentioned by @Wraith2, in SqlClient DbDataReader.GetFieldValue<T>() can be used to get a Stream or TextReader, exactly in the same way as currently returned by the dedicated GetStream() and GetTextReader() methods. We should implement this as well.

This has the advantage of also providing async column access via GetFieldValueAsync() without adding new GetStreamAsync() and GetTextReaderAsync().

@bgrainger, assign this to me, please. I will take a care of it.

[bgrainger]

I added GetFieldValueAsync tests here: 04efd32