jetpack-mu-wpcom: migrate coming-soon to package

[samiff]

Proposed changes:

• Branched from Add plugin jetpack-mu-wpcom-plugin #28479
• Migrates the coming-soon feature to the jetpack-mu-wpcom package, that was previously added to the plugin version in mu-wpcom: add coming-soon code from ETK #27815
• Adds initialization for the coming-soon feature.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Internal PT: p9dueE-6jY-p2

Does this pull request change what data or activity we track or use?

No.

Testing instructions:

The jetpack-mu-wpcom package or plugin is not used in production anywhere just yet, so merging this should be safe and follow-up PRs can be used to address any issues that popup when implementing on wpcom/WoA. The bulk of the code in this PR is migration from #27815

I did do some quick testing on simple wpcom:

• bin/jetpack-downloader test jetpack-mu-wpcom-plugin update/migrate-coming-soon-to-package
• Will need to uncomment the require to mu-wpcom-plugin.php in 0-wpcom-jetpack-loader.php.
• If not merged/deployed yet, will want to patch with changes in ETK: update loading coming-soon check wp-calypso#72901
• Can then add some error logging to prod/full-site-editing-plugin.php & class-jetpack-mu-wpcom.php vendor package file to verify which codepath is being loaded.
• Enable the Coming Soon feature and test it out. Check PHP error log, there should be no redeclaration or other errors.

[github-actions]

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Mu Wpcom plugin plugin:

• Next scheduled release: March 7, 2023.
• Scheduled code freeze: February 27, 2023.

[anomiex]

Code is basically the same as what was removed in #28445, let's do it.

[sdixon194]

Was just about to approve as well - also functionally tested on wpcom by requiring the new plugin and confirmed it loads and works fine.