Allow reading Exif metadata from AVIF images

[Shnatsel]

The image crate uses mp4parse to decode AVIF images. I'd like to implement Exif metadata extraction from AVIF there, but mp4parse does not expose a function to retrieve the Exif metadata in AvifContext.

There's already Rust code for writing Exif into AVIF files which can be used as a reference:

https://github.com/kornelski/avif-serialize/blob/f4f4f96e28d0bd8da9e085ec90220708cc49bbbd/src/lib.rs#L214-L231

Adding such a function should be quite straightforward for someone already familiar with the format. I'm not familiar with it at all, which is why I am opening an issue and not a PR.