XMP metadata read support

[1c3t3a]

XMP is a common metadata format supported by a bunch of the formats that are available in this crate. We can extend the ImageDecoder trait to make it available to extract this metadata.

This is what I would like to work towards:

Read support for XMP metadata in:

• [x] JPEG - ~~implemented in zune-jpeg already, needs to be released.~~ done in #2634
• [x] PNG - implemented in png crate but not wired up to image
• [x] WebP - implemented in the image-webp crate but not wired up to image
• [x] TIFF - implemented in tiff crate but not wired up to image
• [ ] GIF - implemented in the gif crate (PR: https://github.com/image-rs/image-gif/pull/215) but not wired up to image

Is XMP common in other formats?

[fintelia]

The image-webp crate should already support extracting XMP metadata.