Added LedPlugin system for simulating LEDs/Indicators

[jasmeet0915]

🎉 New feature

Closes #3245

Summary

gazebo_led_plugin_demo_final-2025-12-28_23.24.45.mp4

• The plugins allows a user to define an LED using a visual, a light, or both via the <led> element (see example snippet below). Internally the plugin uses the names from the <visual_name> and <light_name> elements with the entitiesFromScopedName() function to get their respective entities

  <!-- LED Descriptions -->
  <led name="emergency_led">
    <visual_name>lamp_red::lamp_red_visual</visual_name> <!-- scoped name of the visual -->
    <light_name>lamp_red::emergency_light</light_name> <!-- scoped name of the visual -->
    <default_state>
      <color>0.3 0.0 0.0 1</color>
      <intensity>0.0</intensity>
    </default_state>
  </led>

• The plugin allows a user to describe different named <mode> in the SDF each of which can contain a bunch of <step> that describes the state of the mode's <active_leds> during that step in terms of <color>, <on_time> and <intensity>. The plugin applies the different steps of a mode sequentially to the required LEDs in PreUpdate():

  <mode name="ready">
    <active_leds>
      <led>ready_led</led>
    </active_leds>
    <step always_on="false">
      <color>0.0 1.0 0.0 1</color>
      <intensity>2.0</intensity>
      <on_time>1.0</on_time>
    </step>
    <step always_on="false">
      <color>0.0 0.3 0.0 1</color>
      <intensity>0.0</intensity>
      <on_time>1.0</on_time>
    </step>
  </mode>

• A change_mode service is advertised by the plugin namespaced with the <led_group_name> or the model name if no LED group name is provided. This service can be requested to trigger a mode change by passing the string name of the required mode.
• The plugin uses the VisualCmd and LightCmd components of the associated visual and light entities of the defined LEDs to set the required color and intensity (intensity is only for light).

Test it

The plugin can be tested with the example world added with this PR (video attached above):

gz sim examples/worlds/led_plugin.sdf

Checklist

• Signed all commits for DCO
• Added a screen capture or video to the PR description that demonstrates the feature
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• Updated Bazel files (if adding new files). Created an issue otherwise.
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