feat(data-events): add ActionScheduler logging and retry reason

[miguelpeixe]

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• Have you followed the Newspack Contributing guideline?
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Changes proposed in this Pull Request:

NPPD-1238

Depends on Automattic/newspack-newsletters#2031.

Implements AS logging and a reason property to retry actions for data events and syncs.

The sync method will return multiple errors, so the logged string will be the joined error messages rather than the usual ->get_error_message(), which returns the first error message from the object.

Automattic/newspack-newsletters#2031 is required because the upsert errors were no longer exposed in favor of a reader-friendly message. The proposed change is to compose a new WP_Error object with all the errors, with the generic reader-friendly message first so it's used in UIs.

How to test the changes in this Pull Request:

1. Change your ESP settings to an invalid access token
2. Register a new reader in a fresh session
3. As an admin, navigate to ActionScheduler page: /wp-admin/admin.php?page=wc-status&tab=action-scheduler
4. Search for newspack_contact_sync_retry
5. Confirm that the latest entry contains reason in the args
6. Wait for the first retry to run and confirm that, once completed, the logs are presented:

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?