Add the ability to auth via certs without storing them in etcd secret

[enxebre]

What type of PR is this?

What this PR does / why we need it:

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 #5198

Special notes for your reviewer:

• cherry-pick candidate

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

[k8s-ci-robot]

Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[enxebre]

cc @bennerv @bryan-cox

[codecov]

Codecov Report

Attention: Patch coverage is 38.46154% with 8 lines in your changes missing coverage. Please review.

Project coverage is 52.65%. Comparing base (9ba44ee) to head (ff70797).

Files with missing lines	Patch %	Lines
azure/scope/identity.go	38.46%	6 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5199      +/-   ##
==========================================
- Coverage   52.66%   52.65%   -0.01%     
==========================================
  Files         273      273              
  Lines       29189    29197       +8     
==========================================
+ Hits        15371    15374       +3     
- Misses      13029    13033       +4     
- Partials      789      790       +1     

☔ View full report in Codecov by Sentry.
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[k8s-ci-robot]

@enxebre: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-cluster-api-provider-azure-verify	ff70797	link	true	/test pull-cluster-api-provider-azure-verify

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[bryan-cox]

/close

in favor of #5200

[k8s-ci-robot]

@bryan-cox: You can't close an active issue/PR unless you authored it or you are a collaborator.

In response to this:

/close

in favor of #5200

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[enxebre]

let's follow up in the other PR
/close