Support disabling alpha channel in the color picker

[peteward]

期望的新功能 (describe the expected new feature)

Hi 👋
The pdfme project, which uses form-render has a requirement to disable alpha channel from the color picker UI. This is supported by rc-color-picker and also the suggested future alternative, @rc-component/color-picker

简述一下使用场景，便于开发者更好理解新功能的必要性 (describe your scenario for us to understand the need)

This would allow you to change the color picker from:

to:

This can be achieved using the enableAlpha prop.

[lhbxs]

Thank you for submitting the issue, which has been merged