[FR] Support GIT source

[JohnPreston]

I would like to be able to retrieve files from a GIT source.

• git+ssh://
• git+https://
• git+http://
• codecommit://

One must agree on:

• When using SSH, I understand that I need to provide the private and public key for the authentication to work.
• When authentication required, I understand that I must provide username/passwords

When using CodeCommit, the IAM role used by the task must provide access to the necessary resources on AWS.
For codecommit, will use https://github.com/aws/git-remote-codecommit as library to retrieve the URL to peform the clone.

[JohnPreston]

For SSH to work, will need #10 to be implemented to ensure key is in place prior to pulling/cloning.