feat: add methods that were in mailchimp data connector

[leogermani]

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:

Brings the updated functionality of preparing merge fields that was present in the Mailchimp data api connector

How to test the changes in this Pull Request:

See Automattic/newspack-plugin#3237

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?

[codecov]

Codecov Report

Attention: Patch coverage is 56.92308% with 28 lines in your changes missing coverage. Please review.

Project coverage is 18.27%. Comparing base (c029516) to head (f34f74a).

Files	Patch %	Lines
...mailchimp/class-newspack-newsletters-mailchimp.php	56.92%	28 Missing ⚠️

Additional details and impacted files

@@                        Coverage Diff                        @@
##             epic/consolidate-data-flows    #1568      +/-   ##
=================================================================
+ Coverage                          17.78%   18.27%   +0.49%     
- Complexity                          2296     2302       +6     
=================================================================
  Files                                 44       44              
  Lines                               8706     8731      +25     
=================================================================
+ Hits                                1548     1596      +48     
+ Misses                              7158     7135      -23     

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[dkoo]

Working as expected here too 👍