feat: Add Infrahub Jinja2 transform for VRF/L3VNI configuration (#21)

- Add GraphQL query vrf_intent.gql: queries InfraVRFDeviceAssignment by
  device name, returning device-specific RD, VRF attributes (name,
  description, l3vni), and both device-level and VRF-level import/export
  route targets for fallback logic
- Add Jinja2 template vrf_yang.j2: renders a JSON array of VRF config
  objects with RD fallback (device → VRF-level) and RT merge logic
  (device-level if present, otherwise VRF-level); all optional
  relationships guarded with 'is defined and is not none' checks
- Update .infrahub.yml: register vrf_intent query and vrf_yang_transform
- Add unit test fixtures for leaf3: input.json (mock GraphQL response)
  and output.json (expected rendered JSON array)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

infrahub/transforms/tests/vrf_yang/test.yml

@@ -0,0 +1,15 @@
+---
+version: "1.0"
+infrahub_tests:
+  - resource: Jinja2Transform
+    resource_name: vrf_yang_transform
+    tests:
+      - name: smoke_check
+        spec:
+          kind: jinja2-transform-smoke
+      - name: render_leaf3
+        spec:
+          kind: jinja2-transform-unit-render
+          directory: infrahub/transforms/tests/vrf_yang
+          input: input.json
+          output: output.json