azure: support system-assigned managed identity

[patrickdillon]

System-assigned managed identities can be used in the same manner as user-assigned managed identities, simply by leaving the ID unset.

What type of PR is this?

/kind feature

What this PR does / why we need it:
Allows system-assigned managed identities as well as the currently supported user-assigned managed identity.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

One thing that's ugly about this is that it currently uses the UserAssignedMSI identity type. I figured I would float this simple fix to start the conversation. Happy to add a new type if that is better.

TODOs:

• squashed commits
• includes documentation
• adds unit tests
• cherry-pick candidate

Release note:

Adds support for system-assigned managed identities.

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @patrickdillon. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

Regular contributors should join the org to skip this step.

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[mboersma]

/ok-to-test

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 44.43%. Comparing base (0666062) to head (18b0d21).
⚠️ Report is 40 commits behind head on main.

Files with missing lines	Patch %	Lines
azure/credential_cache.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6152      +/-   ##
==========================================
- Coverage   44.43%   44.43%   -0.01%     
==========================================
  Files         280      280              
  Lines       25367    25371       +4     
==========================================
+ Hits        11272    11273       +1     
- Misses      13283    13286       +3     
  Partials      812      812              

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[patrickdillon]

/hold

testing of this has so far been unsuccessful