Feat(eos_cli_config_gen): Implement ipv6 virtual source nat vrfs

[gmuloc]

Enhancement summary

Add

ipv6 virtual source-nat vrf <vrf>

support to eos_cli_config_gen

Which component of AVD is impacted

eos_cli_config_gen

Use case example

Required for #3936

Describe the solution you would like

Add a key to https://github.com/aristanetworks/avd/blob/devel/python-avd/pyavd/_eos_cli_config_gen/schema/schema_fragments/virtual_source_nat_vrfs.schema.yml#L24

virtual_source_nat_vrfs
  - name: <vrf>
     ip_address: <IPv4 address>
     ipv6_address: <IPv6 address> <<<<<<<<<<<<< new key

When both ip_address and ipv6_address are present, both CLI should be rendered for the VRF

Describe alternatives you have considered

No response

Additional context

No response

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