Form validation message customization

## Prerequisites

- [x] I have searched for similar features requests in both open and closed tickets and cannot find a duplicate.
- [x] The feature is still missing in the latest stable version of Elementor ( Elementor Pro. )

**What problem is your feature request going to solve? Please describe.**
The form widget is customizable but we cannot change the message displayed when there's an empty field. Being able to customize this message and the look of it would boost up our consistency and designs.

**Describe the solution you'd like**
Elementor (Pro or not, I don't care) must have the option to change the message, like for example the option to change de form sent sucessfully message. Also, It woulb be cool to change the position of the popup shown and make it display over the field in a custom color and like normal text, for example.

**Describe alternatives you've considered**
Custom code or other plugins

**Additional context**