Fixes #21241

Proposed changes:

• This PR includes:
  • The initial foundation added here: Forms: hidden fields #40329 (allows adding hidden fields via the inspector). Modifications to that include:
  • Creating a button to add new fields (allows for cases when the value field is blank)
  • Adding the hidden fields to form HTML if the value is blank for any name / value hidden field pairs, as well as a new set of filter, which allows third party integration for dynamically added values.
  • Warning messaging if duplicate names are spotted (but not preventing this from happening)
  • An initial framework to allow using hidden fields within the Salesforce form variation for the organization ID. An earlier commit included allowing for the actual hidden field, but this is pared back for now so functionality remains as it was (to simplify this initial hidden fields PR).

Additional notes:

• See https://github.com/Automattic/vulcan/issues/601 for more information on possible next steps in terms of extending hidden fields and, how that can tie in to other functionality.
• In terms of using hidden fields on more than one form on a post / page, this is currently staying as a 'to do', until form reusability steps related to unique form IDs are in place.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

#21241
See also https://github.com/Automattic/vulcan/issues/601

Does this pull request change what data or activity we track or use?

No.

Testing instructions:

On a self-hosted or WoA site, apply this PR via the Jetpack Beta plugin or locally. On Simple, with a sandboxed Simple site apply the changes using the command in the generated comment below.

• Create a new post and add a form block - any form variation will do, but for ease you can use the Contact Form variation.
• Once the form is added, select the form block and notice in the inspector there is a new section for hidden fields.
• You should be able to add new name / value pairs with the Add button.
• If you duplicate a name, you should see a warning message 'on blur' (when clicking elsewhere out of the name field). This should disappear once the duplicate name is changed.
• If you test using a pre-defined hidden field with the name '[post_id]', this should remove the value field, and replace that with the form ID.

Form response data and exporting data:

• After submitting a form that includes hidden fields, view the feedback response at wp-admin/edit.php?post_type=feedback and ensure the hidden field data is there.
• Try exporting the data via CSV, and Google sheets. In both cases, the hidden fields names should be prefixed with '85_', which will place them after the more important form data such as name, email, etc, but before Consent and IP address.

Testing form filters:

• On self-hosted / WoA:
• Create two new sets of hidden fields. Name one [post_id] and name another 'Test', with a value 'Demo'.
• Using a code snippet plugin / functionality plugin, add the following code:

add_filter( 'jetpack_contact_form_modify_hidden_fields', 'custom_modify_hidden_field_both', 10, 3 );

function custom_modify_hidden_field_both( $updated_field, $hidden_field ) {
    if ( isset( $updated_field['value'] ) && $updated_field['value'] === 'Demo' ) {
        $updated_field['name'] = 'Custom Name';
        $updated_field['value'] = 'Custom Value';
    }

    return $updated_field;
}

• Activate the snippet, then on the front-end of the site fill out the demo form with the added hidden fields.
• Check the form responses at wp-admin/edit.php?post_type=feedback - you should see one entry for [post_id] with the post id, and you should see one entry with the name 'Custom Name' and value of 'Custom Value'. There should be no entry for 'Test' with the value 'Demo'.
• Add the following snippet:

add_filter( 'jetpack_contact_form_modify_hidden_field_name', 'custom_modify_hidden_field_name', 10, 3 );

function custom_modify_hidden_field_name( $field_name, $field_value, $hidden_field ) {
    if ( $field_value === '1771' ) { // replace 1771 with the post ID that your form is included in.
        return 'No post ID';
    }

    return $field_name;
}

• Activate the snippet, then on the front-end of the site fill out the demo form with the added hidden fields.
• Check the form responses at wp-admin/edit.php?post_type=feedback - you should see one entry for 'No post ID' with the post id, as well as the changed values from the previous values (or the original name / value pair values for those hidden fields if the other snippet is deactivated).