Colorpicker format and validation

With Admin v1.10.30.1, the [colorpicker](https://learn.getgrav.org/17/forms/blueprints/fields-available#colorpicker-field) seemingly supports two formats: Hex and RGBA, but not RGB.

For example, when a blueprint uses the color `rgb(255, 138, 128)`, it renders fine:


When you click the field, the colorpicker-composes interprets it as a hex-value and transforms it into `#ff8a80`:


If you then click the transparent-button, it gets interpreted back to RGB with an alpha-channel set to 0:


Whereas it should be `rgba(255, 138, 128, 1.00)`. But why isn't RGB simply supported? With a simple list of colors, like

```yaml
colors:
- name: White
value: "rgb(250, 250, 250)"
- name: Red
value: "rgb(255, 138, 128)"
- name: Orange
value: "rgb(255, 209, 128)"
```

these will not fit the pattern-validation, and when trying to save settings in a blueprint it will practically go through each one, transforming them to the nearest Hex. Is the solution #921 not sufficient?

Start with the colorpicker field documentation and reproduce the blueprint example using rgb(...) values. Compare the current behavior with the solution referenced in #921, then verify that RGB values pass validation, are not converted to hex, and transparent values use the expected rgba(..., 1.00) form.

Written by the indexing model from the issue text.