jetpack Test ExPlat implementation

Proposed changes:

This PR exists solely to help testing #37958. For more information, check the PR description there. The changes on this PR are the same as the ones in #37958 plus the implementation of an experiment in My Jetpack title.

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?

Testing instructions:

Spin up a JN site with Jetpack Beta and make sure "Enable Sandbox Access" is on. Do not choose the feature branch yet
Connect your site to WPCOM
Go to Settings > Jetpack Constants (or /wp-admin/options-general.php?page=companion_settings) and point JETPACK__SANDBOX_DOMAIN to your sandbox address
Then, visit /wp-admin/admin.php?page=jetpack-beta&plugin=jetpack and search for the feature branch add/test-explat
Head to 21843-explat-experiment and follow the instructions on how to manually assign a variation with a bookmarklet
Try assigning treatment to your anonymous user and visit my Jetpack. Confirm that the title is "Explore all Jetpack Products"
Assign control and go back to My Jetpack; the title should say "Discover all Jetpack Products"
You can also check the requests in the network tab, filtering for "assignment" and checking the response matches the below (for control, the value of the property explat_test_jetpack_plugin_explat_implementation in the variations object should be null)

Jun 20 '24 20:06 IanRamosC

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

:white_check_mark: Include a description of your PR changes.
:white_check_mark: Add a "[Status]" label (In Progress, Needs Team Review, ...).
:white_check_mark: Add testing instructions.
:white_check_mark: Specify whether this PR includes any changes to data or privacy.
:white_check_mark: Add changelog entries to affected projects

This comment will be updated as you work on your PR and make changes. If you think that some of those checks are not needed for your PR, please explain why you think so. Thanks for cooperation :robot:

The e2e test report can be found here. Please note that it can take a few minutes after the e2e tests checks are complete for the report to be available.

Once your PR is ready for review, check one last time that all required checks appearing at the bottom of this PR are passing or skipped. Then, add the "[Status] Needs Team Review" label and ask someone from your team review the code. Once reviewed, it can then be merged. If you need an extra review from someone familiar with the codebase, you can update the labels from "[Status] Needs Team Review" to "[Status] Needs Review", and in that case Jetpack Approvers will do a final review of your PR.

Jetpack plugin:

The Jetpack plugin has different release cadences depending on the platform:

WordPress.com Simple releases happen daily.
WoA releases happen weekly.
Releases to self-hosted sites happen monthly. The next release is scheduled for August 6, 2024 (scheduled code freeze on August 5, 2024).