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/output.json

@@ -0,0 +1,15 @@
+[
+  {
+    "description": "VRF gold - L3 VXLAN with symmetric IRB",
+    "export_targets": [
+      "1:100001"
+    ],
+    "import_targets": [
+      "1:100001"
+    ],
+    "l3vni": 100001,
+    "redistribute_connected": true,
+    "route_distinguisher": "10.0.250.13:1",
+    "vrf_name": "gold"
+  }
+]