Add an adapter for Campaign Monitor

[jackmarchant]

Hey 👋
We've needed to migrate from SendGrid to Campaign Monitor, so we've written an adapter for it, and largely based it off the SendGrid adapter.

Currently, it supports classic email and smart email, but Smart email is not documented or tested well yet. I wanted to check first to see if this is something you want to add before spending too much time on it, happy to update it before merging too.

https://www.campaignmonitor.com/api/transactional/#send-classic-email
https://www.campaignmonitor.com/api/transactional/#send-smart-email

[maymillerricci]

@jackmarchant Thanks for this! At this point our preference is for new adapters to be their own separate libraries. See for example the others under https://github.com/thoughtbot/bamboo#available-adapters. Can you implement this as its own library? Then we can add a link to your new one to the list as well. 😄

[jackmarchant]

Yes, no problem at all! I just thought I’d submit a PR here in case. I’ll submit a PR when it’s ready

Thank you!

[jackmarchant]

#480
@maymillerricci