feat: Add legacy pattern support for EVPN ethernet segments (17.12 compatibility)

[aitestino]

Problem

PR #355 addressed the EVPN ethernet-segment-num test failures by excluding tests on platforms/versions that don't support it. However, the implementation doesn't follow the provider's established pattern for version compatibility.

The YANG path for EVPN ethernet segments differs between IOS-XE versions:

• 17.12: Cisco-IOS-XE-l2vpn:evpn/ethernet-segment
• 17.15+: Cisco-IOS-XE-l2vpn:evpn/ethernet-segment-choice/evpn-num-case/ethernet-segment-num/ethernet-segment

Current Implementation

The current interface_port_channel.yaml only supports the 17.15+ path:

- yang_name: Cisco-IOS-XE-l2vpn:evpn/ethernet-segment-choice/evpn-num-case/ethernet-segment-num/ethernet-segment
  xpath: Cisco-IOS-XE-l2vpn:evpn/ethernet-segment-num/ethernet-segment
  tf_name: evpn_ethernet_segments

Proposed Solution

Follow the provider's established "legacy" pattern used in other resources (e.g., route_map.yaml):

1. Keep the current attribute for 17.15+
2. Add a new _legacy suffixed attribute for 17.12
3. Use test_tags to control which versions use which attribute

Example Implementation

Current attribute for 17.15+

  - yang_name: Cisco-IOS-XE-l2vpn:evpn/ethernet-segment-choice/evpn-num-case/ethernet-segment-num/ethernet-segment
    xpath: Cisco-IOS-XE-l2vpn:evpn/ethernet-segment-num/ethernet-segment
    tf_name: evpn_ethernet_segments
    test_tags: [IOSXE1715]
    type: List
    attributes:
      - yang_name: es-value
        id: true
        example: 1

  # New legacy attribute for 17.12
  - yang_name: Cisco-IOS-XE-l2vpn:evpn/ethernet-segment
    xpath: Cisco-IOS-XE-l2vpn:evpn/ethernet-segment
    tf_name: evpn_ethernet_segments_legacy
    test_tags: [IOSXE1712]
    type: List
    attributes:
      - yang_name: es-value
        id: true
        example: 1

Tasks

• Add evpn_ethernet_segments_legacy attribute to gen/definitions/interface_port_channel.yaml
• Regenerate the provider code
• Update tests to use appropriate attribute based on version
• Remove the exclude_test: true workaround from PR fix: resolve issues with integration tests for port-channels and port-channel subinterfaces #355
• Test on both 17.12 and 17.15 environments

Acceptance Criteria

• EVPN ethernet segment configuration works on 17.15+ using the standard attribute
• EVPN ethernet segment configuration works on 17.12 using the legacy attribute
• Tests pass on all pipeline environments
• No breaking changes to existing configurations

[aitestino]

@ChristopherJHart