feat(auth-mailchimp-sync): added script for syncing exisiting mailchimp contacts

[dackers86]

fixes: #637

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[google-cla]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
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[codecov-commenter]

Codecov Report

Merging #464 into next will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             next     #464   +/-   ##
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  Coverage   41.58%   41.58%           
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  Files          30       30           
  Lines        1128     1128           
  Branches      154      154           
=======================================
  Hits          469      469           
  Misses        653      653           
  Partials        6        6           

[google-cla]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
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[dackers86]

Closing as issue closed and move to mailchimp/Firebase#29