[python] support external type #8767
                
     Draft
            
            
          
  Add this suggestion to a batch that can be applied as a single commit.
  This suggestion is invalid because no changes were made to the code.
  Suggestions cannot be applied while the pull request is closed.
  Suggestions cannot be applied while viewing a subset of changes.
  Only one suggestion per line can be applied in a batch.
  Add this suggestion to a batch that can be applied as a single commit.
  Applying suggestions on deleted lines is not supported.
  You must change the existing code in this line in order to create a valid suggestion.
  Outdated suggestions cannot be applied.
  This suggestion has been applied or marked resolved.
  Suggestions cannot be applied from pending reviews.
  Suggestions cannot be applied on multi-line comments.
  Suggestions cannot be applied while the pull request is queued to merge.
  Suggestion cannot be applied right now. Please check back later.
  
    
  
    
fix #8709
azure-core is ready: Azure/azure-sdk-for-python#43051
Since this repo can't keep generated code while we have to add customized code in _patch.py (e.g. here) for test, so we have to add test case in autorest.python repo: https://github.com/Azure/autorest.python/pull/3246/files#diff-0934ce9d44a15f3f825714c63c3c5f13bc242a99dec49efade2742385e2d7d78.
pending on: