Enhancement: support IDNS on URL form input validations

### Impacted plugin

Jetpack

### What

Add validation support for IDNS, example: https://παράδειγμα.test

### How

perhaps we can abstract URL validation functionality to a shared helper function somewhere as it'd seem quite useful to have around (and tested). I tried looking to see if we already did that somewhere, though the closest I found was using [FILTER_VALIDATE_URL](https://www.php.net/manual/en/filter.filters.validate.php) which isn't perfect as it will not work for internationalized domain names (IDNs).

Quoted from https://github.com/Automattic/jetpack/pull/27138#pullrequestreview-1177703733

Start by reviewing the closed, merged pull request #40490 and the existing URL validation based on FILTER_VALIDATE_URL. Done means the example https://παράδειγμα.test is accepted through the relevant form validation, with coverage for internationalized domain names and any shared helper behavior.

Written by the indexing model from the issue text.