🌱 Add script for API diff check

[EmilienM]

What this PR does / why we need it:

• Add target to Makefile to run make apidiff
• Add the script itself, copied from other platforms

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 #1860

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[EmilienM]

/cc mdbooth

[EmilienM]

mhh I need to find a way to skip it for v1alpha1 stuffs.

[EmilienM]

We agreed the following:

• Add a non voting job to run apidiff
• If the job fails, ignore failures if it's because of a change in our go api until we figure things out with our codebase.

[mdbooth]

/approve

Wouldn't it be nice if we had a tool like Zuul which could test both this PR and https://github.com/kubernetes/test-infra/pull/32644 together?

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mdbooth

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/kubernetes-sigs/cluster-api-provider-openstack/blob/main/OWNERS)~~ [mdbooth] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[jichenjc]

/lgtm