Add missing schema-validation built-in check and config

Fix the builtin-schema-validation test failure by creating:
- Built-in check that wraps kubeconform template for schema validation
- Test config file that enables the schema-validation check

This completes the implementation started in commit 0304239 which added
the test and deprecation message but missed these files.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: janisz

e2etests/testdata/schema-validation-config.yaml

@@ -0,0 +1,4 @@
+checks:
+  addAllBuiltIn: false
+  include:
+    - "schema-validation"

pkg/builtinchecks/yamls/schema-validation.yaml

@@ -0,0 +1,10 @@
+name: "schema-validation"
+description: "Validate Kubernetes resources against their schemas using kubeconform"
+remediation: "Fix the resource to conform to the Kubernetes API schema"
+scope:
+  objectKinds:
+    - Any
+template: "kubeconform"
+params:
+  strict: true
+  ignoreMissingSchemas: true