[Enhancement] Add MTU support for Loopback, VLAN, Port-Channel, and Tunnel interfaces

[aitestino]

Summary

The provider currently only supports interface-level mtu configuration for Ethernet interfaces. However, the Cisco IOS-XE YANG model (Cisco-IOS-XE-interfaces.yang) shows that Loopback, VLAN, Port-Channel, and Tunnel interfaces also support interface-level MTU configuration via the interface-common-grouping.

Current State

Supported:

• iosxe_interface_ethernet - has mtu attribute (gen/definitions/interface_ethernet.yaml:32)

Not Supported:

• iosxe_interface_loopback - missing mtu attribute
• iosxe_interface_vlan - missing mtu attribute
• iosxe_interface_port_channel - missing mtu attribute
• iosxe_interface_tunnel - missing mtu attribute

Note: These interfaces currently only expose protocol-specific MTU options (ipv6/mtu for all types, ip/mtu for tunnels), but not the interface-level mtu leaf.

Desired State

Add the interface-level mtu attribute to all interface types, matching what's available in the YANG model:

- yang_name: mtu
  example: 1500

YANG Model Reference

The interface-common-grouping in Cisco-IOS-XE-interfaces.yang (line 3868) defines:

leaf mtu {
  description "Set the interface Maximum Transmission Unit (MTU)";
  type uint16 {
    range "64..18000";
  }
}

This grouping is used by:

• list Loopback (uses interface-common-grouping)
• list Vlan (uses interface-common-grouping)
• list Port-channel (uses interface-common-grouping)
• list Tunnel (uses interface-common-grouping)
• list GigabitEthernet (uses interface-common-grouping) ✅ already implemented

Use Case

Users need to configure MTU on non-Ethernet interfaces for various networking scenarios:

• Loopback interfaces with jumbo frames for internal routing
• VLAN interfaces (SVIs) with custom MTU for inter-VLAN routing
• Port-channel interfaces with adjusted MTU for link aggregation
• Tunnel interfaces with optimized MTU for overlay networks

Proposed Changes

Update the following provider definition files to add the mtu attribute:

1. gen/definitions/interface_loopback.yaml
2. gen/definitions/interface_vlan.yaml
3. gen/definitions/interface_port_channel.yaml
4. gen/definitions/interface_tunnel.yaml

Each should include:

- yang_name: mtu
  example: 1500

This would bring these interfaces to feature parity with the Ethernet interface implementation.