feat: add support for gke clusterclass

[salasberryfin]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR adds support for managed clusters (GKE) provisioning using ClusterClass, implementing the template types required to satisfy the CAPI contract.

New templates for GKE cluster provisioning via ClusterClass are added to ./templates:

• Standard GKE cluster: ./templates/cluster-template-gke-clusterclass.yaml and ./templates/cluster-template-gke-topology.yaml
• Autopilot cluster: ./templates/cluster-template-gke-autopilot-clusterclass.yaml and ./templates/cluster-template-gke-autopilot-topology

I've tried these two templates with the current CAPI and CAPG and both clusters are provisioned successfully.

The CAPI contract for using ClusterClass requires that a bootstrap object is passed in the infrastructure definition, which is a problem when provisioning managed clusters, which do not need cluster bootstrapping. Other providers have used different alternatives to bypass this requirement: CAPZ does reference Kubeadm (which then creates a dependency on the provider) and CAPA implements its own version of a bootstrap provider for EKS. I think the simplest solution is to create a "semi-dummy" GKEConfig controller that will report the Ready status to core CAPI when GKEMachinePools are available. This controller does not apply changes other than reporting the readiness of the cluster's infrastructure. When using GKE Autopilot there's no need to reference a bootstrap object as underlying infrastructure is completely managed by GCP, as can be seen in the new templates.

Pending: adding e2e tests

Existing GKE E2E tests are not running because of issues with GCP permissions which we're trying to get fixed here. There is a separate PR to enable these tests on CI. Considering GKE is still an experimental feature of CAPG, I'd suggest we try to get this feature added and, in parallel, work on having E2E permissions fixed. Specially considering that this adds lots of lines of new code, having to keep this PR up to date for a long time may become quite complex.

Which issue(s) this PR fixes:
Fixes #1387

Special notes for your reviewer:

Changes affect the experimental GKE feature which hopefully helps with publishing changes in the API.

NOTE: After bumping to CAPI v1.10 this PR required a number of changes. Since some of these have already been applied in existing GKE (non-clusterclass) logic, there was some overlap of code and I've switched some methods to functions so that they can be reused across different resources (i.e. GCPManagedMachinePool and GCPManagedMachinePoolTemplate have equivalent ValidateUpdate logic and it would not make sense to maintain separate code for each resource). Some of the changes also apply to using individual validation of fields in the specification, rather than doing bulk validation of multiple fields at the same time: this means the code is more verbose but I consider it is overall more clear and definitely makes it more reusable and scalable down the line.

Follow-ups:

• Add documentation to book.

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

Experimental feature GKE now supports provisioning via ClusterClass

[k8s-ci-robot]

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[salasberryfin]

/retest

[salasberryfin]

/retest

[salasberryfin]

/retest

[salasberryfin]

cc @damdo @cpanato @alexander-demicev

[alexander-demicev]

thanks a lot for working on this. This PR has been open for while, in my opinion we should merge it as is and improve in follow up PRs if needed. The feature is experimental anyway.

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: alexander-demicev, salasberryfin

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[alexander-demicev]

/remove-lifecycle-stale

[salasberryfin]

/hold

[salasberryfin]

/retest

[alexander-demicev]

/lgtm

[salasberryfin]

/unhold