Fix: Add pagination handling to CloudFront distribution listing

[ChadReitsma]

The CloudFront API will only return a maximum of 100 items per request.

If a user has more than 100 distributions they cannot test or purge the CDN from within WP Admin if the distribution item is not returned within the first 100 results.

These changes make the associated functions follow pagination markers to create an array of all distributions (not just the first 100).

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 35 lines in your changes missing coverage. Please review.
✅ Project coverage is 1.52%. Comparing base (8f8a85c) to head (41ef249).

Files with missing lines	Patch %	Lines
CdnEngine_Mirror_CloudFront.php	0.00%	35 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             master   #1238      +/-   ##
===========================================
- Coverage      1.52%   1.52%   -0.01%     
- Complexity    19773   19782       +9     
===========================================
  Files           634     634              
  Lines         99758   99791      +33     
===========================================
  Hits           1521    1521              
- Misses        98237   98270      +33     

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[ChadReitsma]

Hey guys, is there something I can do to help speed up the merging process for this commit? It's a pretty simple fix.