Next generation backend plugins

Backend plugins have seen a major overhaul.

These changes will apply to all backend plugins:

1. Backend plugins are now auto-discovered. Your custom backend plugin will automatically show up on the settings page without altering FillPdfSettingsForm.
2. In your plugin annotation, you may now provide a label, a description and a weight. (implemented in #3044666: Autodetect and present all available backend plugins)

Your own custom backend plugins, if you created any, will continue to work. In order to use new features (like plugin inspection and self-contained configuration) and to prepare your plugin for the next major FillPDF version 8.x-5.x, you should however update your custom plugin:

• Backend plugins now extend PdfBackendBase (implemented in #3060086: Introduce new PdfBackend plugins to replace legacy FillPdfBackend and BackendService plugins).
• The plugin's populateWithFieldData() method has been replaced by mergeFile() which takes:
  • a File object representing the PDF template, while before it took the whole FillPDFForm.
  • an array of FieldMapping objects (TextFieldMapping and ImageFieldMapping) as second parameter, while before it took a complex array of field mapping data.

Before:

/**
 * @Plugin(
 *   id = "local_service"
 * )
 */
class LocalService implements FillPdfBackendPluginInterface, ContainerFactoryPluginInterface {

Now:

/**
 * @PdfBackend(
 *   id = "local_service",
 *   label = @Translation("FillPDF LocalServer"),
 *   description = @Translation("Network-accessible, self-installed PDF API. You will need a VPS or dedicated server."),
 *   weight = 5
 * )
 */
class LocalService extends PdfBackendBase implements ContainerFactoryPluginInterface {