Migrate custom Key Vault resource to azidentity

[thomas11]

• Use KV in the azure-in-azure test to test different auth scopes
• Migrate custom KV resource to azidentity. Need to preserve the previous one due to special authorizer.
• Fall back to previous custom KV resources to use correct authorizer

[github-actions]

Does the PR have any schema changes?

Looking good! No breaking changes found.
No new resources/functions.

[codecov]

Codecov Report

Attention: Patch coverage is 25.80645% with 46 lines in your changes missing coverage. Please review.

Project coverage is 59.91%. Comparing base (8a11520) to head (b45a6a7).

Files with missing lines	Patch %	Lines
...ources/customresources/custom_keyvault_autorest.go	17.14%	27 Missing and 2 partials ⚠️
...r/pkg/resources/customresources/custom_keyvault.go	0.00%	13 Missing ⚠️
...r/pkg/resources/customresources/customresources.go	71.42%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3664      +/-   ##
==========================================
- Coverage   60.15%   59.91%   -0.25%     
==========================================
  Files          68       69       +1     
  Lines       10878    10928      +50     
==========================================
+ Hits         6544     6547       +3     
- Misses       3831     3877      +46     
- Partials      503      504       +1     

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