ESP sync - all plan names & bugfix

[adekbadek]

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:

1. Changes the ESP field names Membership Plan (added in feat(esp-sync): sync membership data regardless of subscription #3353) to Membership Plans, to handle multiple plan names
2. Fixes a bug reported here: feat(esp-sync): sync membership data regardless of subscription #3353 (comment)

Note that 1. is not a breaking change since #3353 is not released. This PR will have to be merged to alpha ("alpha-hotfix").

How to test the changes in this Pull Request:

1. In the Engagement wizard, "Email Service Provider (ESP) Advanced Settings" section, ensure "Sync contacts to ESP" is toggled on
2. Create a new membership, with a new contact – observe it's synced to the ESP, with Membership Plans field present
3. Add a plan to the membership, observe both plans' names are synced in the field

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?

[adekbadek]

It looks like these fields are planned for removal in #3411, so the fixes here won't apply.