Code refactoring & Datadog provider

[Ketouem]

⚠️ @builtinnya A CircleCI build configuration is included with this PR, in order to activate it you'll need to go to https://circleci.com and enable it. Don't hesitate to ping me if you're struggling with this step.

• Datadog events are now pushed to Slack
• Code has been refactored (both lambda and test program)
• Makefile is provided
• Unit tests are provided (can be launched with make test
• Proper CI build that run the tests and ensure that the pushed zip package is the most up to date one
• README has a CircleCI status badge (needs activation by an admin cc @builtinnya ?)
• Circumvent TF issue (Using ${path.module} in aws_lambda_function filename results in incorrect "difference" between user machines hashicorp/terraform#7613) for aws_lambda_function.filename with ${path.module}

[builtinnya]

I'm sorry for being really late to respond 😢
I have not noticed your PR (I overlooked the notification email of it).
It contains too big changes in a PR, so please let me take some time to look into it!