Add validation webhooks (GKE part 7)

[richardchen331]

/kind feature

What this PR does / why we need it:
This change adds validation webhooks for GCPManagedCluster, GCPManagedControlPlane, and GCPManagedMachinePool.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Relates #512

Special notes for your reviewer:

Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

Add validation webhooks for **GCPManagedCluster**, **GCPManagedControlPlane**, and **GCPManagedMachinePool**

[k8s-ci-robot]

Hi @richardchen331. 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.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

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/test-infra repository.

[cpanato]

/retest

[richardchen331]

/retest

[k8s-ci-robot]

@richardchen331: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

In response to this:

/retest

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/test-infra repository.

[richardchen331]

@richardcase @cpanato could you help trigger another retest? I don't think the error is related to the code change here. Thanks!

[cpanato]

/ok-to-test
/test pull-cluster-api-provider-gcp-e2e-test

[richardcase]

This is looking good, thanks.

One quick question, apart from that i think we are good to go.

[richardcase]

/lgtm

[richardcase]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: richardcase, richardchen331

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