Contact Form: Support custom field validation by themes/plugins

#### Steps to reproduce the issue

The `validate()` function of `Grunion_Contact_Form_Field` has no filters so it isn't easy to write custom validation logic for a field - for example adding your own validation for a url or making the email validation stricker/looser

#### What I expected

A filer

#### What happened instead

There was no filter

Start by locating the Grunion_Contact_Form_Field class and its validate() function in the Jetpack codebase. Review nearby validation behavior and existing filter patterns, then determine how a filter can support custom URL or email validation. Done means themes and plugins can alter field validation without modifying the class.

Written by the indexing model from the issue text.