Enhancement: Forms - add pattern attribute to forms URL field

### Impacted plugin

Jetpack

### What

It could be helpful in some cases to include the pattern attribute to the `field-url` field, in a way that can be customizable in the editor.

### How

This was suggested in the form of a request regarding how to add the attribute, in a forum thread: https://wordpress.org/support/topic/add-validation-pattern-on-wpjetpack-field-url/

### Why add this

The pattern attribute can add a level of validation to the `field-url` field, ensuring only URLs fitting specific patterns are accepted. See the [Mozilla developer docs](https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/pattern) for more information.

Start by locating Jetpack Forms' implementation of the field-url field and the editor configuration that controls its attributes. Review the linked forum request and the MDN pattern documentation, then identify the relevant tests or add coverage; done means the field accepts a customizable pattern and validation behavior is verified.

Written by the indexing model from the issue text.