Feat(eos_designs): Add speed option for default_interfaces

[pvinci-arista]

Enhancement summary

When using the default_interfaces: model, it makes sense to also be able to define corresponding interface speed.

Which component of AVD is impacted

eos_designs

Use case example

please detail your use case

Describe the solution you would like

default_interfaces:
  - types: [ l3leaf ]
    platforms: [ 720XP-48 ]
    uplink_interfaces: [ Ethernet53/1, Ethernet54/1 ]
    uplink_speed: forced 100G
    ```


### Describe alternatives you have considered

Manually configure the interface speed under the node type.

### Additional context

_No response_

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