feat(esp-sync): queue data events sync to run once

[miguelpeixe]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Since #3616, data events are grouped and handled in the same execution. This PR leverages that change to also optimize multiple syncs that different data events may trigger.

To restrict that behavior to data events only, this PR implements the feature from #3383 so we can be aware of whether we're running under a data event or not.

Syncs performed under data events should now only run once per email, containing the latest payload.

For debugging, the sync contexts are grouped to be logged, which can look like this:

To ensure we always have the most recent data, when running on shutdown the most recent contact data is fetched using a new get_contact_data() method, which was decoupled from the sync_contact() method.

How to test the changes in this Pull Request:

1. Make sure you have a RAS setup with a membership plan and a subscription product activating it
2. In a fresh session, go through the Checkout Button flow and complete the subscription purchase
3. Confirm via logs that a bunch of data events gets dispatched (membership_status_active, membership_saved, reader_registered)
4. Confirm that only one Newspack_Newsletters_Contacts::upsert is logged
5. Confirm the contact is added to your ESP

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

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