Make output's max_power and light's gamma_correct templatable

**Describe the problem you have/What new integration you would like**

Make output's max_power and light's gamma_correct templatable

**Please describe your use case for this integration and alternatives you've tried:**

If max_power and gamma_correct was templatable, it would allow us to calibrate it directly from HomeAssistant. One could create a slider for each and adjust them dynamically. It would really ease light calibration for instance. That would be how I'd use it, but I think there are a lot of scenarios where it would be handy to make these properties templatable.

**Additional context**

I have no idea if the same could be achieved by other means right now. I've tried to use espHome's API but I couldn't figure how to do it properly.

I've created an input_number in HomeAssistant, and corresponding sensor in espHome with a on_value event, but I couldn't find how to set gamma_correct or max_power in this event.

I assume - perhaps wrongly - that if gamma_correct and max_power were templatable, it would be easier to do it.

No source file or test is named. Start by locating the ESPHome output component's max_power and light component's gamma_correct handling, then review how on_value and the Home Assistant API expose configurable values. Done means both properties can be driven dynamically from Home Assistant without breaking existing static configuration.

Written by the indexing model from the issue text.