Fixing without an issue since it's such a small bug.

Issue #3060086 by Pancho, wizonesolutions: Introduce new PdfBackend plugins to replace legacy FillPdfBackend and BackendService plugins

Issue #3046257 by Pancho, wizonesolutions: Move EntityHelper::getFormFields() to the actual FillPdfForm entity and deprecate the 'fillpdf.entity_helper' service

Fix fails along the way and create a config schema so that I can test
properly.

Also fixes file usage handling for FillPdfForm.

Works for FillPDF forms now but not for generated PDFs.

I'm getting an error when I actually try to populate/download the PDF. There is probably
something wrong with the Request object I'm returning. But this is closer.

Routes are still missing, but they haven't been made yet, so that's why.

There's lots of changes here. I should have committed more often.
Anyway, the gist of it is:
  - General code conversion to Drupal 8 patterns. I've taken shortcuts here and there but mostly avoid that. No idea on the quality. Hopefully, someone can give me some feedback.
  - Two new entity types, FillPdfForm and FillPdfFormField. These
    correspond to the old {fillpdf_forms} and {fillpdf_fields}
    tables. Now they're finally entities, so Views support will
    be possible for those of you who wanted it. I was finally forced to
    do this, and I'm glad.
  - PDF-filling methods are now PLUGINS!!! This means they all
    implement FillPdfBackendPluginInterface, and this basically
    achieves the same thing I was trying to do with the failed
    pdf_forms project. Anyone who wants to add their own
    backend can already do it; the fillpdf_service_backend
    configuration variable is used to select the correct backend.
    It should match the annotation ID. Look at
    FillPdfServiceFillPdfBackend to get an idea of how to structure the
    plugin.
  - Better use of actual entity references. FillPdfForms store a
    reference to their underlying Files instead of just an ID. Same with
    FillPdfFormField -> FillPdfForm.