[azpysdk] Add --md flag to azpysdk apistub to generate api.md

[swathipil]

Description

Part of Azure/azure-sdk-tools#12851.

Add an --md flag to azpysdk apistub. When passed, the command will additionally run Export-APIViewMarkdown.ps1 on the generated JSON token file to produce api.md.

azpysdk apistub . --md

Potential Implementation

Changes to eng/tools/azure-sdk-tools/azpysdk/apistub.py:

1. Add --md flag (action="store_true", default False) in the register() method.
2. After successfully writing the JSON token file, if --md is set, invoke via subprocess:

   subprocess.run(
       ["pwsh", "eng/common/scripts/Export-APIViewMarkdown.ps1",
        "-TokenJsonPath", <json_file>, "-OutputPath", <out_dir>],
       check=True
   )

   If pwsh is not available, fail with a clear error.
3. Place api.md in the same output directory as the token file (under <dest-dir>/<package-name>/ if --dest-dir is provided).

Deliverables

• azpysdk apistub supports a new --md to create an api.md file, which is output to the same directory as the APIView token file.
• If pwsh is not available, the utility fails gracefully with a clear error message.