feat(netbox): Add NetBox API client for fabric intent retrieval

Implements FabricNetBoxClient using pynetbox to fetch:
- Device and interface data
- BGP sessions and peer groups (plugin)
- L2VPN/EVPN VNI mappings
- VRF and route target configuration
- MLAG custom fields

Relates to #5

pyproject.toml

@@ -7,6 +7,7 @@ requires-python = ">=3.12"
 dependencies = [
     "click>=8.1.0",
     "pygnmi>=0.8.0",
+    "pynetbox>=7.0.0",
     "rich>=13.0.0",
 ]