Support OIDC file token authentication

[thomas11]

One typical way of authenticating via OIDC is that the identity provider supplies the OIDC token in a file. This provider already had support for this built in to the auth stack, but due to an oversight, validation wouldn't consider it and the provider would error before attempting to authenticate. This PR fixed this oversight and resolves #3373.

pulumi/registry#4790 updates the docs accordingly.

[github-actions]

Does the PR have any schema changes?

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.87%. Comparing base (78e0d82) to head (b13fc1a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3382      +/-   ##
==========================================
+ Coverage   56.82%   56.87%   +0.04%     
==========================================
  Files          66       66              
  Lines        8083     8092       +9     
==========================================
+ Hits         4593     4602       +9     
  Misses       3055     3055              
  Partials      435      435              

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[danielrbradley]

Core change looks okay, but I think we need to update the provider config in the schem, and update the docs too: https://www.pulumi.com/registry/packages/azure-native/installation-configuration/

[thomas11]

Core change looks okay, but I think we need to update the provider config in the schem, and update the docs too: https://www.pulumi.com/registry/packages/azure-native/installation-configuration/

Registry PR is already open, see PR description