Feat(eos_cli_config_gen): Add support for IPv6 Helper Addresses on VLAN interfaces

[jrecchia1029]

Enhancement summary

Add support for configuring ipv6_helper addresses on VLAN interfaces.

Which component of AVD is impacted

eos_cli_config_gen

Use case example

It may be necessary to add IPv6 helper addresses in campus networks that use IPv6.

Describe the solution you would like

Add the ipv6_helpers key and values to the supported keys under eos_cli_config_gen's vlan_interfaces data model.

vlan_interfaces:
  - name: <str>
    ipv6_helpers:
      -  ip_helper: <str>
         link_address: <str>
         local_interface: <str>
         source_address: <str>
         vrf: <str>

Describe alternatives you have considered

The eos_cli key in the vlan_interfaces dictionary is a current way to configure this.

Additional context

No response

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[jrecchia1029]

Already supported by ipv6_dhcp_relay_destinations key