feat: add support for mailchimp mandrill

[navinkarkera]

Description

Adds support for Mandrill app for Mailchimp in addition to Braze for transactional emails.

Private-ref: BB-8780.

Testing considerations

I have tested this on mailchimp but it will be helpful if someone with Braze account can test this out (to make sure refactoring did not break anything)

Post-review

Squash commits into discrete sets of changes

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Thanks for the pull request, @navinkarkera!

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