Feat(eos_designs): support mst pvst border as a node default

[pvinci-arista]

Enhancement summary

Add a key such as spanning_tree_mst_pvst_boundary: <bool> so that it can be grouped with the other spanning tree options in the defaults section of a node_type.

Which component of AVD is impacted

eos_designs

Use case example

Certain hardware platforms may not support the number of active spanning tree instances needed for PVST and a large number of configured VLANs. In this case, the spanning tree msp pvst boundary configuration is used.

Describe the solution you would like

Add a key such as spanning_tree_mst_pvst_boundary: <bool> so that it can be grouped with the other spanning tree options in the defaults section of a node_type.

Describe alternatives you have considered

This can be implemented today using:

    #spanning_tree_mode: mstp
    #spanning_tree_priority: 16384
    structured_config:
      spanning_tree:
        mst:
          pvst_border: true

Additional context

No response

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