feat: Add Infrahub Jinja2 transform for MLAG configuration (#22)

- Add GraphQL query mlag_intent.gql: queries InfraMlagPeerConfig by
  device name, returning local IP, peer address, heartbeat peer IP,
  MLAG domain (domain-id, virtual-mac, heartbeat VRF, dual-primary
  detection settings, peer/iBGP VLANs, peer devices), local interface
  SVI name, and peer-link LAG name
- Add Jinja2 template mlag_yang.j2: renders a JSON object targeting
  Arista-native YANG path /arista-mlag-augments:mlag/config; returns
  empty array [] for devices with no MLAG config (spines); all optional
  relationship accesses guarded with 'is defined and is not none' checks
- Update .infrahub.yml: register mlag_intent query and mlag_yang_transform
- Add unit test fixtures for leaf1 (full MLAG config) and spine1 (empty
  response → []) following jinja2-transform-unit-render format

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

infrahub/transforms/tests/mlag_yang/spine1/input.json

@@ -0,0 +1,7 @@
+{
+  "data": {
+    "InfraMlagPeerConfig": {
+      "edges": []
+    }
+  }
+}

infrahub/transforms/tests/mlag_yang/spine1/output.json

@@ -0,0 +1 @@
+[]