Feat(eos_designs): Add shutdown support for mgmt interfaces in eos_designs

[ankudinov]

Enhancement summary

Implement an eos_designs equivalent for:

custom_structured_configuration_mgmt_interfaces:  
  shutdown: <bool>

The use case was not specified in the original bb issue. However I'd expect either chassis or an inband management case where OOB is disabled for security reasons.

#6534 / #6627 implements some changes to the mgmt model. While there is no direct relationship, we can consider implementing another node level feature to control shutdown:

nodes:
    - name: mgmt-interface-dhcp
      mgmt_shutdown: [ interface-name-list ]
      id: 201

We could also consider implementing a more generic knob under avd_design_future where we list the mgmt interface names to disabled and allow setting it for a corresponding group / host.

Which component of AVD is impacted

eos_designs

Use case example

see above

Describe the solution you would like

see above

Describe alternatives you have considered

No response

Additional context

No response

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