Create a `Format` enum for file format/mime type

[ok-nick]

It's not clear in the API which file formats/mime types are supported. We also often end up string matching when dealing with different file formats. I think we can make some big improvements by introducing a Format enum similar to image::ImageFormat.

We can provide methods such as:

• c2pa::format_from_path -> Format::from_path
• Format::from_extension
• Format::from_mime_type
• Format::mime_type
• Format::format
• Format::extensions
• Format::can_read
• Format::can_write

Source: https://github.com/contentauth/c2pa-rs/pull/1260#discussion_r2229607467