feat(ci): add CRD schema generation for kubeconform validation

Add scripts to extract OpenAPI v3 schemas from CRD definitions in all
kustomize overlays and write JSON schema files to ci/crd-schemas/ for
kubeconform validation. This allows kubeconform to validate CRD instances
(Elasticsearch, Kibana, CNPG Cluster, VictoriaMetrics, etc.) instead of
skipping or erroring on them.

- ci/generate-crd-schemas.py: extracts schemas from CRD YAML on stdin
- ci/generate-crd-schemas.sh: iterates overlays, pipes to Python script
- ci/validate-apps.sh, ci/validate-clusters.sh: add local schema-location fallback
- Makefile: add generate-schemas target
- ci/crd-schemas/: 67 generated schemas (cert-manager, cnpg, elastic, victoriametrics, argocd, gateway)
- .pre-commit-config.yaml: raise large-file limit to 2 MB for generated schemas

Run `make generate-schemas` to regenerate after CRD version changes.

ci/crd-schemas/postgresql.cnpg.io/imagecatalog_v1.json

@@ -0,0 +1,62 @@
+{
+  "$schema": "http://json-schema.org/schema#",
+  "description": "ImageCatalog is the Schema for the imagecatalogs API",
+  "properties": {
+    "apiVersion": {
+      "description": "APIVersion defines the versioned schema of this representation of an object.\nServers should convert recognized schemas to the latest internal value, and\nmay reject unrecognized values.\nMore info: https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#resources",
+      "type": "string"
+    },
+    "kind": {
+      "description": "Kind is a string value representing the REST resource this object represents.\nServers may infer this from the endpoint the client submits requests to.\nCannot be updated.\nIn CamelCase.\nMore info: https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#types-kinds",
+      "type": "string"
+    },
+    "metadata": {
+      "type": "object"
+    },
+    "spec": {
+      "description": "Specification of the desired behavior of the ImageCatalog.\nMore info: https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#spec-and-status",
+      "properties": {
+        "images": {
+          "description": "List of CatalogImages available in the catalog",
+          "items": {
+            "description": "CatalogImage defines the image and major version",
+            "properties": {
+              "image": {
+                "description": "The image reference",
+                "type": "string"
+              },
+              "major": {
+                "description": "The PostgreSQL major version of the image. Must be unique within the catalog.",
+                "minimum": 10,
+                "type": "integer"
+              }
+            },
+            "required": [
+              "image",
+              "major"
+            ],
+            "type": "object"
+          },
+          "maxItems": 8,
+          "minItems": 1,
+          "type": "array",
+          "x-kubernetes-validations": [
+            {
+              "message": "Images must have unique major versions",
+              "rule": "self.all(e, self.filter(f, f.major==e.major).size() == 1)"
+            }
+          ]
+        }
+      },
+      "required": [
+        "images"
+      ],
+      "type": "object"
+    }
+  },
+  "required": [
+    "metadata",
+    "spec"
+  ],
+  "type": "object"
+}