fix: handle update event for the github customer resource handler

[xuezhma]

Describe your changes

• added UPDATE handler support for sagemaker-templates GitHub repository custom resource:
  • CodeConnectionArn changes: updates CodeBuild source credentials
  • S3BucketObjectKey changes (seed code updates): creates branch and pull request with new code
  • repository_owner changes: creates new repository under new owner with seed code, keeps old repository intact
• added codebuild:DeleteSourceCredentials permission to sagemaker-templates GitHub repository Lambda role

Issue ticket number and link

Checklist before requesting a review

• I updated CHANGELOG.MD with a description of my changes
• If the change was to a module, I ran the code validation script (scripts/validate.sh)
• If the change was to a module, I have added thorough tests
• If the change was to a module, I have added/updated the module's README.md
• If a module was added, I added a reference to the module to the repository's README.md
• I verified that my code deploys successfully using seedfarmer apply

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.