Jetpack Sync: Add 'jetpack_package_versions' to Sync option whitelist

[fgiannar]

Adds jetpack_package_versions option to Sync options' whitelist.
Note that since this is a Connection dependant option, there is a fallback in place in order to update the option on WPCOM via a REST request, in case Sync is not present or enabled.

Proposed changes

Sync package:

• Automattic\Jetpack\Sync\Defaults: Add jetpack_package_versions to Sync options whitelist
• Automattic\Jetpack\Sync\Data_Settings: Add jetpack_package_versions to the minimum set of Sync options required by Sync itself

Connection package:

• Automattic\Jetpack\Connection\Package_Version_Tracker: Update update_package_versions_option method to only update the jetpack_package_version option on WPCOM via a REST request if Sync is not present or enabled

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

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

No

Testing instructions:

D136887-code

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WordPress.com Simple site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin, and enable the update/jetpack-package-versions-add-to-sync branch.

• To test on Simple, run the following command on your sandbox:

  bin/jetpack-downloader test jetpack update/jetpack-package-versions-add-to-sync
  

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

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[darssen]

Thanks for working on this @fgiannar.

I reviewed and approved the WPCOM side of it.

I was able to test with the instructions and things worked well for me for my local and a new JN site.

The code in this PR looks also fine as far as I can tell but I'd like someone else more familiar with the connection part to take a look and approve.

[sergeymitr]

I tested different scenarios, everything works like a charm 👍
Disabling Sync did make it switch to the endpoint, so the fallback also works great.