This task is to create and maintain a feature branch with the latest work for the purpose of testing the combine in progress client side media feature.

Note: client side media currently does not work in playground (see WordPress/wordpress-playground#2954),

The testing PR: #75189

Testing instructions:

1. Build the branch and enable the client side media experiment
2. test uploading one or more images
3. Verify correct subsized images are available on the server
4. Verify srcset is output on the front end
5. See PR for detailed testing instructions and full listed of implemented capabilities