Use the `draft` binary for GitHub workflow file generation

[peterbom]

This extension does not currently use Draft's generate-workflow command to generate GitHub workflow files.

The reasoning is that Draft is not sufficiently configurable to support workflow generation comparable to Automated Deployments. Specifically, it does not support:

• specifying the workflow name (will be addressed by #324)
• specifying different resource groups for ACR and cluster (will be addressed by #324)
• specifying the cluster namespace to deploy to (will be addressed by #324)
• manifest files in a location other than manifests within the project root (see: #325), making it unsuitable for repositories containing more than one service
• specifying multiple manifest files (see: #325)

After these are addressed in Draft, the workflow generation can be simplified by invoking the Draft binary and removing the template files.