Add Flatcar Container Linux sysext templates

[sayanchowdhury]

What type of PR is this?
/kind feature

What this PR does / why we need it:

This PR adds a new cluster template for Flatcar based on systemd-sysexts. The advantages of the implementation is that the clusters can directly consume the vanilla Flatcar SIG images, instead of the Flatcar-K8S images built through image-builder.

The PR is currently in draft stage, as I'm looking into adding the tests for the PR but opening it for early comments.

cc @pothos

Special notes for your reviewer:

• cherry-pick candidate

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

Add new flavor `flatcar-sysext` for Flatcar Container Linux cluster deployments.

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @sayanchowdhury. 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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.73%. Comparing base (d0f78e0) to head (5fbbe08).
Report is 2 commits behind head on main.

❗ Current head 5fbbe08 differs from pull request most recent head 2581dc8. Consider uploading reports for the commit 2581dc8 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4575      +/-   ##
==========================================
+ Coverage   62.71%   62.73%   +0.01%     
==========================================
  Files         192      192              
  Lines       15641    15487     -154     
==========================================
- Hits         9810     9715      -95     
+ Misses       5148     5105      -43     
+ Partials      683      667      -16     

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

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[sayanchowdhury]

/test pull-cluster-api-provider-azure-e2e-optional

[k8s-ci-robot]

@sayanchowdhury: The following tests 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-e2e-optional	953f850	link	false	/test pull-cluster-api-provider-azure-e2e-optional
pull-cluster-api-provider-azure-e2e-aks	2581dc8	link	true	/test pull-cluster-api-provider-azure-e2e-aks
pull-cluster-api-provider-azure-windows-with-ci-artifacts	2581dc8	link	false	/test pull-cluster-api-provider-azure-windows-with-ci-artifacts
pull-cluster-api-provider-azure-conformance	2581dc8	link	false	/test pull-cluster-api-provider-azure-conformance
pull-cluster-api-provider-azure-conformance-ipv6-with-ci-artifacts	2581dc8	link	false	/test pull-cluster-api-provider-azure-conformance-ipv6-with-ci-artifacts
pull-cluster-api-provider-azure-e2e	2581dc8	link	true	/test pull-cluster-api-provider-azure-e2e
pull-cluster-api-provider-azure-apiversion-upgrade	2581dc8	link	false	/test pull-cluster-api-provider-azure-apiversion-upgrade
pull-cluster-api-provider-azure-conformance-with-ci-artifacts	2581dc8	link	false	/test pull-cluster-api-provider-azure-conformance-with-ci-artifacts
pull-cluster-api-provider-azure-conformance-dual-stack-with-ci-artifacts	2581dc8	link	false	/test pull-cluster-api-provider-azure-conformance-dual-stack-with-ci-artifacts
pull-cluster-api-provider-azure-windows-custom-builds	2581dc8	link	false	/test pull-cluster-api-provider-azure-windows-custom-builds
pull-cluster-api-provider-azure-conformance-custom-builds	2581dc8	link	false	/test pull-cluster-api-provider-azure-conformance-custom-builds

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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