[AKS]Support associating an existing user assigned identity for MSI cluster

[norshtein]

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This checklist is used to make sure that common guidelines for a pull request are followed.

General Guidelines

• Have you run azdev style <YOUR_EXT> locally? (pip install azdev required)
• Have you run python scripts/ci/test_index.py -q locally?

For new extensions:

• My extension description/summary conforms to the Extension Summary Guidelines.

About Extension Publish

There is a pipeline to automatically build, upload and publish extension wheels.
Once your PR is merged into master branch, a new PR will be created to update src/index.json automatically.
The precondition is to put your code inside this repo and upgrade the version in the PR but do not modify src/index.json.

[arrownj]

LGTM. However there is another PR which also update HISTORY.md and version number, this will cause a merge conflict

[yungezz]

hi @norshtein could you pls resolve conflict for next step?

[norshtein]

Hi @arrownj , @yungezz , I've fixed the merge conflict, can we merge the PR?

[zqingqing1]

lgtm