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.

此問題涉及 Jetpack Forms 及其 "Choice (Radio)" 欄位，但未指出檔案、測試或程式碼進入點。先找出表單欄位的實作並檢視連結的支援討論；當使用者建立聯絡表單時可以指定一個單選選項預設為選取狀態，即表示完成。

由索引模型根據 Issue 內容生成。