Improvements to Image CDN package

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](https://github.com/Automattic/jetpack/pull/29561#discussion_r1151022382)

Start by reading the main PR #29561 and the linked discussion comment, then inspect the Image CDN classes for the unchecked class-member visibility review. Confirm the remaining checklist items are resolved and that the package changes remain consistent with the main PR.

Written by the indexing model from the issue text.