Add color format to REST API validation

[spacedmonkey]

Add validation for color as a formatted type in REST API requests.

[chrisvanpatten]

I dig this a lot but it should probably be named "hex color" or something like that, to distinguish it from other valid color types (rgb, rgba, hsl, etc)

[TimothyBlynJacobs]

This looks great! I agree with @chrisvanpatten about naming to make it clear this is a hex color format.

[spacedmonkey]

Thanks for the feedback. In 49270.4.diff, I change naming to hex-color and added tests.

You can also posts comments on PR ​here.

[TimothyBlynJacobs]

In 47450:

REST API: Introduce "hex-color" JSON Schema format.

Props spacedmonkey, chrisvanpatten.
Fixes #49270.