Provide an overload for JsonWriter.WriteValue to accept Span<byte> input

The recently introduced Span<T> offers advantages, specifically obviating the need to deal with byte buffers in most cases. I started to use it in my code, too, but the C# driver's JsonWriter.WriteValue method won't accept Span<byte> and a span has to be converted to a byte array first, which results in a heap allocation: span.ToArray().

The driver should support this new .NET type so that we can use it without such unnecessary allocation penalties.