Improvements to Image CDN package

[dilirity]

List things left to do from the main PR https://github.com/Automattic/jetpack/pull/29561.

Quick summary

More things to do:

• [x] Configure webpack to build and minify the js file
• [x] Remove photon code from Jetpack and depend on the plugin (separate PR)
• [x] Sync the module status between Jetpack and Boost
• [x] Update WPCOM to adapt to these changes
• [x] Change functions.photon.php to a utility class and move the functions from root namespace.
• [x] Add deprecated warnings for old functions
• [x] Move compatibility to Boost and pin it to older Jetpack versions
• [x] Autoload files in Image CDN.
• [x] Cleanup and update docblock comments
• [ ] Review class member visibility
• [x] Address regex issues discovered by @anomiex on the main pr #29561
• [ ] Address issue in this comment

[github-actions[bot]]

This issue has been marked as stale. This happened because:

• It has been inactive for the past 6 months.
• It hasn’t been labeled `[Pri] BLOCKER`, `[Pri] High`, `[Type] Feature Request`, `[Type] Enhancement`, `[Type] Janitorial`, `Good For Community`, `[Type] Good First Bug`, etc.

No further action is needed. But it's worth checking if this ticket has clear reproduction steps and it is still reproducible. Feel free to close this issue if you think it's not valid anymore — if you do, please add a brief explanation.