Support for AWS Pinpoint

[fubarhouse]

My employer is in an interesting position where we are unable to provision simple email service in a local region. Because of this we are exploring the use of Pinpoint - which is supported in our region and shares IAM permissions with simple email service.

This PR adds an optional flag -p to target a similar pinpoint service. Efforts have been made to make this as much like the SES relay namespace as possible - swapping out the relevant parts to be compatible with the dissimilar structure and requirements.

It maintains full compatibility with the SES relay and tests have been reproduced for this relay.

We would appreciate any feedback you can provide.

docker run fubarhouse/aws-smtp-relay:pinpoint --help

Note that we haven't tested this in a real-world situation however this application has quite a remarkable amount of tests which are passing. I'll leave this as open (out of draft) and subject to @blueimp's discretion as to when it is approved/merged.

[fubarhouse]

Please re-review it when you get a chance 👍 All requested changes have been revisited and completed.

[blueimp]

Thanks for the changes @fubarhouse

I did another more thorough review and sent you an accompanying pull request for your feature branch: https://github.com/fubarhouse/aws-smtp-relay/pull/2

The changes I implemented on top of your branch:

• Use an interface to implement the ses|pinpoint relay clients.
• Provide factory functions to create a relay client.
• Use the smtpd.Handler function signature for relay.Client.Send.
• Refactor the logging functionality and move it into a separate package.
• Move arguments parsing into a separate function.
• Add links to Pinpoint Email API and SMTP interface.
• Fix the usage documentation.

[blueimp]

Thanks again for your contribution @fubarhouse! ☺️

[fubarhouse]

Not a problem - it has been an interesting project to be involved with 🙂