Rethink on how to adopt and maintain the customization codes

[MaryGao]

Things to consider:

• In SDK repo, how many customization patterns we have?
  // Mary: we need to understand current status.
• Should we migrate existing patterns to new recommendation? Or should codegen consider major existing cases?
  // Mary: we need to decide how to support existing cases.
• Customization v2 should be our recommandation.
• How to ensure the new customization process could take effective? We always propose new ideas if no check there, no ones would follow.
  // Mary: we need to ensure new practice should be followed in future.
• In codegen, some flags like clear-out-dir or generate-metadata or some ARM assumption would impact the generation. Could we simplify this?
  // Mary: it's time to refactor codegen logic but not urgent.

Relevant issues:

• Restore support for generate-metadata false #3454
• Metadata.json generated in wrony place if we have customized generated folder #3325

[MaryGao]

Customization information would be collected here: https://microsoftapc-my.sharepoint.com/:x:/g/personal/marygao_microsoft_com/EeuSZxGIK1dOuQ8oxHJwFjsBGSsKuqAyEAsLti3Pe7mDcA?e=7r6lUr