[Feedback Widget] Set initial Custom Field values programmatically

Support for setting custom field & regular field values via Feedback Widget programmatically would optimize user feedback.

Some fields might be confusing for users to fill and are not viable as metadata fields, but by auto-filling these with default values would speed up the process.

Example:

// ... 
const currentPage = 'https://example.com/current-page';
const userClickedButtonName = "button-42";

window.postMessage({
    target: 'FeaturebaseWidget',
    data: { 
      action: 'openFeedbackWidget',
      setBoard: 'yourboardname', // optional - preselect a board
      customFieldsInitialValues: {
        '123abc456def': currentPage,
        '321cba654fed': userClickedButtonName
      }
    }
})
// ...