Add support for 4 gray-scale in waveshare_epaper 7.50inV2alt

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

Waveshare 7.5inch e-paper 4 gray scale not fully supported.
https://www.waveshare.com/product/displays/7.5inch-e-paper.htm

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

I would like to use the 4 gray-scale to write some text other than black or white.

**Additional context**

Current configuration:

`display:
- platform: waveshare_epaper
id: eink_display
cs_pin: GPIO15
dc_pin: GPIO27
reset_pin: GPIO26
busy_pin:
number: GPIO25
inverted: true
model: 7.50inV2alt`

The following attribute is not recognised for this platform: color_palette: 4-gray