Forms: Radio single choice option selected by defaut

### Impacted plugin

Jetpack

### What

Allow users to set a choise selected by default

### How

This was suggested here: https://wordpress.org/support/topic/form-single-choice-radio-default-choice/

Currently, when creating a contact form using Jetpack Forms and including the "Choice (Radio)" field type, there is no option to set one of the choices as selected by default. This enhancement would allow users to designate a default selection for the radio button choices.

The issue concerns Jetpack Forms and its "Choice (Radio)" field, but names no files, tests, or code entry point. Start by locating the form field implementation and reviewing the linked support discussion; done means users can designate one radio choice as selected by default when creating a contact form.

Written by the indexing model from the issue text.