Add Private Link support

[robinkb]

What type of PR is this?

/kind feature

What this PR does / why we need it:

From: #3747 (comment)

The feature is intended for clusters with private networking, it adds option to deploy private link for internal API server load balancer, which enables users to access private clusters from anywhere in Azure simply by creating private endpoints to it (CAPZ got support for private endpoints #3044). More details in the issue linked below.

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 #3400

Special notes for your reviewer:

This PR was originally submitted in #3747, but unfortunately it got stalled due to time constraints. I have gone through the comments in the original PR, and applied what still seemed relevant to the revision in this PR. Special thanks to @nprokopic for development of the feature.

TODOs:

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

Release note:

Add Private Links support

[k8s-ci-robot]

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[k8s-ci-robot]

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[k8s-ci-robot]

Hi @robinkb. 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.

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[nprokopic]

/ok-to-test

Thanks for reviving this :)

FYI stale bot closed the issue #3400, probably a good idea to check with CAPZ folks if the issue should be reopened.

[codecov]

Codecov Report

❌ Patch coverage is 34.74114% with 479 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.64%. Comparing base (bd1e799) to head (8d93ffc).
⚠️ Report is 26 commits behind head on main.

Files with missing lines	Patch %	Lines
...rivatelinks/mock_privatelinks/privatelinks_mock.go	0.00%	370 Missing ⚠️
azure/services/privatelinks/client.go	0.00%	44 Missing ⚠️
azure/services/privatelinks/spec.go	80.55%	14 Missing and 7 partials ⚠️
azure/services/privatelinks/privatelinks.go	67.44%	13 Missing and 1 partial ⚠️
util/futures/getter.go	0.00%	9 Missing ⚠️
azure/scope/cluster.go	75.75%	6 Missing and 2 partials ⚠️
...zure/services/privateendpoints/privateendpoints.go	0.00%	5 Missing ⚠️
azure/scope/managedcontrolplane.go	0.00%	2 Missing ⚠️
azure/services/subnets/spec.go	0.00%	1 Missing and 1 partial ⚠️
controllers/azurecluster_reconciler.go	50.00%	1 Missing and 1 partial ⚠️
... and 1 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6274      +/-   ##
==========================================
- Coverage   43.88%   43.64%   -0.24%     
==========================================
  Files         289      293       +4     
  Lines       25351    26077     +726     
==========================================
+ Hits        11125    11382     +257     
- Misses      13448    13904     +456     
- Partials      778      791      +13     

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