JSON: use Encoding.CreateTranscodingStream when reading JSON in materializer

[maumar]

          The end result seems to be thing: 

new MemoryStream(Encoding.UTF8.GetBytes(dataReader.GetFieldValue<string>(0)))

Regardless of ADO.NET drivers where we can get UTF8 directly out (#31216), we can optimize this by using Encoding.CreateTranscodingStream over a simple Stream to read the string as raw bytes:

Span<byte> buffer = // buffer to read into
var jsonValue = "{}";
MemoryMarshal.Cast<char, byte>(jsonValue.AsSpan(0, Math.Min(jsonValue.Length, buffer.Length / 2))).CopyTo(buffer);

Thanks to @NinoFloris for pointing out all the above.

Originally posted by @roji in #31160 (comment)