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.

该问题涉及 Jetpack Forms 及其 "Choice (Radio)" 字段，但没有指出文件、测试或代码入口点。首先定位表单字段的实现并查看链接的支持讨论；当用户创建联系表单时可以指定一个单选项默认为选中状态，即可视为完成。

由索引模型根据 Issue 内容生成。