Allow Stream and TextReader to be retrieved by GetFieldValue and GetFieldValueAsync

[roji]

As per discussion in dotnet/runtime#28596 (comment), there are currently no async counterparts to DbDataReader.Get{Stream,TextReader}. Rather than adding new methods, it seems better to just allow these types to be returned from DbDataReader.GetFieldValue (and Async).

Added tests to @bgrainger's AdoNetApiTest for Stream: mysql-net/AdoNetApiTest#145

/cc @Wraith2

[cheenamalhotra]

Acknowledged for future.

[David-Engel]

Seems like this was covered/completed by the implementation in #1019