[FR]: Add main entity field at card's root level

[luixal]

Is your feature request related to a problem? Please describe.

I'm trying to use paper-buttons-row inside a button-card template (I need 7 identical cards, only entity changes). Like this:

button_card_templates:

  irrigation_button:
    entity: binary_sensor.irrigation_unlimited_c1_z1
    icon: |
      ...
    custom_fields:

      subtext: |
        ...

      row_buttons:
        card:
          type: custom:paper-buttons-row
          # ADD GLOBAL ENTITY FIELD HERE
          preset: mushroom
          base_config:
            preset: mushroom
            entity: binary_sensor.irrigation_unlimited_c1_z1 # Button-card can't set entity here!
          buttons:
            - icon: mdi:timer-play-outline
              name: |
                {% set next_duration = state_attr(config.entity, 'next_duration') %}
                {% if next_duration %}
                  {{ next_duration.split(':')[1] }}
                {% else %}
                  0
                {% endif %}
            - icon: mdi:play
              layout: icon
            - icon: mdi:cancel
              layout: icon

So button-card can't set the entity id in a custom field, but it does set entity id at a root level.

Describe the solution you'd like

Add a global entity field at card's root level (as most cards have) that is passed down to each button if no other entity is provided.

This:

          type: custom:paper-buttons-row
          entity: binary_sensor.irrgation_unlimited_c1_z1 # global entity field here, passed down to each button
          preset: mushroom
          base_config:
            preset: mushroom
          buttons:
            - icon: mdi:timer-play-outline
            - icon: mdi:play
              layout: icon
            - icon: mdi:cancel
              layout: icon

would work just as this other example:

          type: custom:paper-buttons-row
          preset: mushroom
          base_config:
            preset: mushroom
            entity: binary_sensor.irrigation_unlimited_c1_z1
          buttons:
            - icon: mdi:timer-play-outline
            - icon: mdi:play
              layout: icon
            - icon: mdi:cancel
              layout: icon

Describe alternatives you've considered

I haven't found any way of dinamically set the entity id inside base_config field.

Other solutions like lovelace-gen are too heavy, I think.

Additional context

No response