Add a file watcher for Azure client cert auth

[bryan-cox]

What type of PR is this?
/kind feature

What this PR does / why we need it:
Add a file watcher, utilizing fsnotify, to monitor the Azure client certificate authentication. If the file changes, the pod will need to be restarted since Azure SDK doesn't support hotloading a new certificate yet.

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:
This is a follow up to #5200

• cherry-pick candidate

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

NONE

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @bryan-cox. 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

[bryan-cox]

/retest

[bryan-cox]

/test pull-cluster-api-provider-azure-test

[codecov]

Codecov Report

Attention: Patch coverage is 54.34783% with 21 lines in your changes missing coverage. Please review.

Project coverage is 52.78%. Comparing base (ec18b58) to head (97352d2).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
util/filewatcher/filewatcher.go	57.14%	15 Missing and 3 partials ⚠️
azure/scope/identity.go	25.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5254   +/-   ##
=======================================
  Coverage   52.78%   52.78%           
=======================================
  Files         270      271    +1     
  Lines       29057    29103   +46     
=======================================
+ Hits        15337    15362   +25     
- Misses      12928    12945   +17     
- Partials      792      796    +4     

☔ View full report in Codecov by Sentry.
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[nojnhuh]

Is there some more graceful way we can handle this? I don't remember off the top of my head what kinds of cleanup things we might be doing now in main (or will need to do in the future), but I'd prefer not to have to worry about that at all 😄.

[bryan-cox]

I'm open to alternative suggestions. This is just the pattern we were using in a different project for this specific scenario with Azure SDK not being able to hotload the new cert and requiring the pod to restart in order to use the new cert.

Maybe I'm missing it but I don't see any cleanup things in the main.go. Is that where you were referencing or was it someplace else.

[nojnhuh]

Azure SDK doesn't support hotloading a new certificate yet

What exactly does this mean? CAPZ orchestrates reading the cert from disk and using that to create a TokenCredential, and it's already doing that once in every reconciliation. Do successive calls to azidentity.NewClientCertificateCredential with different certificates not produce working creds?