[eng] pipeline to regenerate all existing packages from Typespec

[msyyc]

To avoid potential breakings or predict potential influence caused by codegen, we may need a pipeline to regenerate all existing packages

[msyyc]

• Every time the PR merged into main branch, the regenerate sdk pipeline https://dev.azure.com/azure-sdk/internal/_build?definitionId=7075 will be triggered. After it done, PR named [Automation] Regenerate SDK based on typespec-python branch main will be updated: [Automation] Regenerate SDK based on typespec-python branch main azure-sdk-for-python#36410.
• the pipeline can also be triggered manually and will also be triggered in midnight every day
• If you want to see the diff before merge a branch (e.g. snake-case-naming-optimization , you can trigger it with the branch name:
  
  After it done, a PR named [Automation] Regenerate SDK based on typespec-python branch {your branch name} will be created (e.g. [Automation] Regenerate SDK based on typespec-python branch snake-case-naming-optimization azure-sdk-for-python#36412). In the created PR, you can see the specific diff caused by your PR in last commit named Regenerate SDK based on typespec-python branch {your branch name} (e.g. Azure/azure-sdk-for-python@378a646)
  nit: please sync with latest main branch before run the pipeline with your branch.