feat: wip custom mailchimp footer

[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:

Closes # .

How to test the changes in this Pull Request:

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

❌ Patch coverage is 20.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 23.91%. Comparing base (bf89e79) to head (a7f957b).
⚠️ Report is 243 commits behind head on trunk.

Files with missing lines	Patch %	Lines
...mailchimp/class-newspack-newsletters-mailchimp.php	20.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##              trunk    #1926   +/-   ##
=========================================
  Coverage     23.90%   23.91%           
- Complexity     2903     2904    +1     
=========================================
  Files            50       50           
  Lines         11192    11196    +4     
=========================================
+ Hits           2676     2677    +1     
- Misses         8516     8519    +3     

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