OpenAI: Update convenience layer for multipart/form-data endpoints per APIView feedback

[annelo-msft]

This PR updates audio, file, and image service methods that use the multipart/form-data content type on requests.

It includes the following:

• Removes BinaryData overloads of these service methods, in favor of Stream overload -- this is so users don't inadvertently load large binary content into memory by calling the wrong overload with a large file.
• Adds FileStream overloads for a bit of convenience when a file is opened from disk -- so users don't have to pass the file name in two places.
• Updates some output APIs that were returning BinaryData that could possibly be large files to return Streams instead.