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/queries/vrf_intent.gql

@@ -0,0 +1,50 @@
+query VrfIntent($device_name: String!) {
+  InfraVRFDeviceAssignment(device__name__value: $device_name) {
+    edges {
+      node {
+        route_distinguisher { value }
+        vrf {
+          node {
+            name { value }
+            description { value }
+            route_distinguisher { value }
+            l3vni {
+              node {
+                vni { value }
+                vni_type { value }
+              }
+            }
+            import_targets {
+              edges {
+                node {
+                  target { value }
+                }
+              }
+            }
+            export_targets {
+              edges {
+                node {
+                  target { value }
+                }
+              }
+            }
+          }
+        }
+        import_targets {
+          edges {
+            node {
+              target { value }
+            }
+          }
+        }
+        export_targets {
+          edges {
+            node {
+              target { value }
+            }
+          }
+        }
+      }
+    }
+  }
+}