This fixes error reporting when something goes wrong with the underlying
backend.

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

Issue #3048134 by Pancho: Properly document $context arrays in FillPdfLinkManipulatorInterface::parseLink()

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

Now with real tokens again instead of the fake D7 token! We look at metdata
and stuff to figure out if it's an image field token! For great profit!

Snuck in a couple other fixes.

Turns out tokens don't get replaced with empty values anyway, so reversing
the replacement order was good enough!

And update the feature description.

And a lot more. Fleshed out some services.

The saving part is almost working. Need to get FillPDF file contexts
being saved properly. Then, I need to ensure that access
checking on the files works like I expect. I will probably have to
implement some hook or another.