Issue #3044666 by Pancho: Autodetect and present all available backend plugins

src/FillPdfBackendManager.php

src/Form/FillPdfSettingsForm.php

src/Plugin/FillPdfBackend/FillPdfServiceFillPdfBackend.php

src/Plugin/FillPdfBackend/LocalFillPdfBackend.php

src/Plugin/FillPdfBackend/LocalService.php

src/Plugin/FillPdfBackend/PdftkFillPdfBackend.php

tests/modules/fillpdf_test/fillpdf_test.module

tests/src/Functional/FillPdfSettingsFormTest.php