Feature Request: Private VLAN (PVLAN) support in the data model

## Feature Request: Private VLAN (PVLAN) support in the data model

### Summary
Add native data model support for Private VLANs (PVLANs) on Cisco NX-OS, covering
the VLAN-level private-vlan configuration, `vlan configuration` mode, and the full
set of interface/port-channel private-vlan modes, mappings, and associations.

### Current Behavior
No data model coverage exists for:
- VLAN private-vlan type (primary / isolated / community)
- VLAN private-vlan association (primary <-> secondaries)
- `vlan configuration <id>` stanzas
- Interface/port-channel private-vlan promiscuous mode and mapping
- Interface/port-channel private-vlan trunk promiscuous mode
- Interface/port-channel private-vlan trunk secondary mode and trunk association
- Interface/port-channel private-vlan host mode and host-association
- Interface/port-channel private-vlan trunk mapping (on promiscuous-trunk AND plain trunk ports)
- Interface/port-channel private-vlan trunk allowed vlan (PVLAN trunk form)
- Host-association combined with an access VLAN on the same port

### Expected Behavior
The data model should allow declaring PVLAN VLAN types/associations, `vlan
configuration` stanzas, and the corresponding interface modes/mappings, so that
applying the model produces the equivalent NX-OS configuration below. Secondary
VLANs should be expressible as a single VLAN, a comma list, or a range.

### Example NX-OS Configuration to Support

VLAN-level:

```text
vlan 100
  private-vlan primary
  private-vlan association 101-102
vlan 101
  private-vlan isolated
vlan 102
  private-vlan community

vlan configuration 101
```

Promiscuous trunk port / port-channel:

```text
interface Ethernet1/1
  switchport mode private-vlan trunk promiscuous
  switchport private-vlan mapping 100 101
  switchport private-vlan trunk allowed vlan 1-4094
  switchport private-vlan mapping trunk 100 101
```

Promiscuous mapping-trunk on a standard trunk port (secondary as a range):

```text
interface Ethernet1/2
  switchport mode trunk
  switchport private-vlan mapping trunk 100 101-102
  switchport trunk allowed vlan 100
```

Trunk secondary port:

```text
interface Ethernet1/3
  switchport mode private-vlan trunk secondary
  switchport private-vlan association trunk 100 101
```

Host port (with and without an access VLAN):

```text
interface Ethernet1/4
  switchport mode private-vlan host
  switchport private-vlan host-association 100 101

interface Ethernet1/5
  switchport private-vlan host-association 100 101
  switchport access vlan 100
```

Promiscuous (non-trunk) mapping with a secondary list:

```text
interface port-channel1
  switchport mode private-vlan promiscuous
  switchport private-vlan mapping 100 101,102
```

### Proposed Data Model (illustrative)

```yaml
nxos:
  devices:
    - name: SWITCH-01
      configuration:
        vlan:
          vlans:
            - id: 100
              private_vlan:
                type: primary
                association: [101, 102]
            - id: 101
              name: Private
              private_vlan:
                type: isolated
            - id: 102
              name: Private-community
              private_vlan:
                type: community
        interfaces:
          ethernets:
            - id: 1/1
              switchport:
                mode: trunk-promiscuous
                private_vlan:
                  mapping: { primary: 100, secondary: 101 }
                  trunk_mapping: { primary: 100, secondary: 101 }
                  trunk_allowed_vlans: 1-4094
            - id: 1/3
              switchport:
                mode: trunk-secondary
                private_vlan:
                  trunk_association: { primary: 100, secondary: 101 }
            - id: 1/4
              switchport:
                mode: host
                private_vlan:
                  host_association: { primary: 100, secondary: 101 }
```

### Additional Context
- Feature prerequisite on device: `feature private-vlan`
- Applies to physical interfaces and port-channels

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Feature Request: Private VLAN (PVLAN) support in the data model #167