Use the `draft` binary for GitHub workflow file generation

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](https://github.com/Azure/draft/pull/324))
- specifying different resource groups for ACR and cluster (will be addressed by [#324](https://github.com/Azure/draft/pull/324))
- specifying the cluster namespace to deploy to (will be addressed by [#324](https://github.com/Azure/draft/pull/324))
- manifest files in a location other than `manifests` within the project root (see: [#325](https://github.com/Azure/draft/issues/325)), making it unsuitable for repositories containing more than one service
- specifying multiple manifest files (see: [#325](https://github.com/Azure/draft/issues/325))

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

Start by locating the extension's current GitHub workflow generation entry point and template files, then review Draft's generate-workflow command and the constraints tracked in #324 and #325. Done means the extension can rely on the Draft binary after those upstream capabilities exist and the redundant templates are removed.

Written by the indexing model from the issue text.