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Introduce GCE-specific addons directory

**What this PR does / why we need it**:

GCE & GKE currently rely on the cluster bringup defined in the `cluster/gce` directory, but there isn't a good way of deploying GCE specific manifests. Currently the 2 approaches are, put it in `/cluster/addons`, which implies it should be generally useful (not GCE specific), or it is synthesized by one of the bash scripts in `cluster/gce`.

This PR introduces a straightforward way to have GCE-specific manifests deployed for GCE & GKE, without the need to pollute the general addon space.

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #53032

**Special notes for your reviewer**:

**Release note**:

```release-note
NONE
```