Migrate the SDK automation codebase out of Alps repository

**Description**
In alignment with the "descend from Alps" initiative, we need to migrate the SDK automation codebase to a new repository. The preferred location
is the 'azure-sdk-tools' GitHub repo, although ADO repository is also another option.

Meanwhile, we aim to conduct a significant refactoring to support various scenarios: including the spec pull request, SDK generation, and local execution.

**Goal**
- Code is migrated to the target repo
- Code is deployable and can be integrated into spec PR pipeline
- Preserve the current functionalities as much as possible
- Code is runnable locally
- Perform legacy code clean up (optional)
- Provide a unified solution