Provide guidance on content_encoding

[leo-barnes]

HEIF supports content_encoding of boxes. I don't think I've ever seen this field used, but it could be very useful to keep down the size of XMP blobs since you could theoretically gzip them.

We should explore what the relevant specs say and provide guidance on this. If we think supporting certain types of content encodings would be useful, we should open issues for the various AVIF implementations to add support for it.