FastLED Gradient support

**Describe the problem you have/What new integration you would like**
I would like to see support in ESPHome for the FastLED library gradient color palettes. See here: https://github.com/FastLED/FastLED/wiki/Gradient-color-palettes

**Please describe your use case for this integration and alternatives you've tried:**
I would like to use this to be able to quickly and maybe on the fly create static color gradients on ledstrips with individial pixel control. For example it starts red at the beginning but gradually changes and it will be blue at the end.

Even better would be to be able to change this on the fly with home assistant, but right now the only possible solution (as far as I know) is to cut up the strip in individual partitions and set the color manually, but that takes more processing power and is not really user friendly.

**Additional context**

None

Read the linked FastLED Gradient color palettes reference and review how ESPHome currently handles individually controlled LED strips. Determine the scope and configuration needed for static and Home Assistant-controlled gradients, then document concrete acceptance criteria for the feature.

Written by the indexing model from the issue text.