feat: metal_vlan_info supports facility based vlans

[bersoare]

vlans can be facility based, but the current metal_vlan_info code doesn't handle that case - it fails since a facility based vlan doesn't have the metro field, which is treated as required by the VLAN_RESPONSE_ATTRIBUTE_MAP.
this PR changes the metro attribute to be optional, and adds a new optional attribute for the facility, so it can handle both cases

        {
            "description": "native vlan",
            "facility": { <<<
                "href": "/metal/v1/facilities/d2a72094-26c9-4372-8d65-051424bc370a",
                "id": "d2a72094-26c9-4372-8d65-051424bc370a"
            },
            "id": "abcd",
            "metro": "",
            "vxlan": 1004
        },
        {
            "description": "Public NAT",
            "facility": "",
            "id": "abcd",
            "metro": { <<<
                "href": "/metal/v1/locations/metros/d3d6b29f-042d-43b7-b3ce-0bf53d5754ca",
                "id": "d3d6b29f-042d-43b7-b3ce-0bf53d5754ca"
            },
            "vxlan": 1004
        },

addresses #195

[bersoare]

rebased

[github-actions]

This PR is included in version 0.8.0 🎉