Refactor(eos_cli_config_gen): Add nested model for `ipv6_nd` for the interfaces

[Shivani-gslab]

Add nested model for ipv6_nd for the interfaces and deprecate old keys. (separate PRs for each interface)
Proposed model:

management_interfaces:
  - name: Management1
    ipv6_enable: true  # already implemented
    ipv6_address_auto_config: true
    ipv6_nd:
      ra:
        rx_accept:
           default_route: true
           route_preference: true

For other interfaces, where ipv6_nd keys are already there, deprecate those keys and add new keys to this nested data-model.

• management_interfaces @Shivani-gslab - Feat(eos_cli_config_gen): Add ipv6_nd support to management_interfaces #6593
• ethernet_interfaces @Vibhu-gslab
• dps_interfaces @Shivani-gslab
• loopback_interfaces
• port_channel_interfaces
• tunnel_interfaces
• vlan_interfaces