Add support for `homeassistant` platform in `light` component

**Describe the problem you have/What new integration you would like**
I would like to request a new feature for the `light` component in ESPHome. Specifically, I want to integrate a light entity (e.g., an RGB light bulb) from Home Assistant into ESPHome, similar to a control panel or mirror of the existing light.

**Please describe your use case for this integration and alternatives you've tried:**
Currently, integrating a Home Assistant light entity into ESPHome requires a messy workaround using multiple `sensor`, `template`, `global`, and other intermediate components. This approach is difficult to maintain, especially when handling brightness, color, effects, etc.
It would be much cleaner and more efficient if ESPHome supported a `homeassistant` platform for the `light` component, similar to how it already supports sensors and switches. This would allow seamless control and synchronization of light states from Home Assistant.

**Additional context**
This would be extremely useful in setups where an ESPHome device is meant to act as a controller or dashboard for other lights already controlled in Home Assistant.

Start by reviewing ESPHome’s existing Home Assistant integrations for sensors and switches, then determine how the light request would cover control and synchronization. The feature would be complete when a Home Assistant light can be represented through the light component, including brightness, color, effects, and state updates.

Written by the indexing model from the issue text.