[sdlf-cicd] github support as an alternative to codecommit

[cnfait]

Issue #, if available:

328 although support here is only for public GitHub.

Description of changes: Replace CodeCommit entirely with GitHub - all SDLF repositories (components, main and team repositories) can now be hosted on GitHub. This does not replace CodeBuild and CodePipeline, GitHub Actions are not used.

Creating repositories is done through CloudFormation third-party resource types: https://github.com/aws-ia/cloudformation-github-resource-providers/tree/main/GitHub-Repositories-Repository

Currently SDLF is quite rigid in terms of setup for GitHub, in part due to limitations of the aforementioned resource types.

• Setup a CodeConnection to GitHub.
• Populate /SDLF/GitHub/CodeConnection in SSM Parameter Store with the ARN of the CodeConnection.
• Put the access token in /SDLF/GitHub/AccessToken in SSM Parameter Store as a secure string.
• Enable the third-party resource type GitHub::Repositories::Repository on CloudFormation Registry. Do not forget to configure it.
• Use -f github when deploying SDLF with deploy.sh.

Resource type configuration example:

{
    "GitHubAccess": {
        "AccessToken": "{{resolve:ssm-secure:/cfn/github/accesstoken:1}}"
    }
}

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.