Introduce new PdfBackend plugins to replace legacy FillPdfBackend and BackendService plugins

There was a bug in FillPDF Service image filling; I think the wrong code from before got pasted, or something. Anyway, there's a particular array format that the XML-RPC call expects. It's even used properly in the legacy plugin. Fixed in new patch. Will test this tomorrow for BC against my implementation that currently uses LocalService directly.

Confirmed parsing works for FillPDF Service. Now checking that my code that uses the LocalService BackendService still works with this patch. I'll likely just leave it alone and port it to the new plugin type later on.

The BackendService LocalService BC layer is perfect. Worked fine with my project.

I found some small things to note:

1. +++ b/src/Plugin/BackendService/LocalService.php
   @@ -20,7 +16,12 @@ use Symfony\Component\DependencyInjection\ContainerInterface;
   +  private $pdfbackend;
   

   Should be $pdfBackend

2. +++ /dev/null
   @@ -1,87 +0,0 @@
   diff --git a/src/FillPdfBackendManager.php b/src/Plugin/LegacyBackendManager.php
   
   diff --git a/src/FillPdfBackendManager.php b/src/Plugin/LegacyBackendManager.php
   similarity index 84%
   
   similarity index 84%
   rename from src/FillPdfBackendManager.php
   
   rename from src/FillPdfBackendManager.php
   rename to src/Plugin/LegacyBackendManager.php
   
   rename to src/Plugin/LegacyBackendManager.php
   index 1eb68cb..e7c1023 100644
   
   index 1eb68cb..e7c1023 100644
   --- a/src/FillPdfBackendManager.php
   
   --- a/src/FillPdfBackendManager.php
   +++ b/src/Plugin/LegacyBackendManager.php
   
   +++ b/src/Plugin/LegacyBackendManager.php
   +++ b/src/Plugin/LegacyBackendManager.php
   @@ -1,6 +1,6 @@
   

   Just leaving a comment in-issue to ensure this rename gets properly applied by patch -p1 (if not using git apply).

3. +++ b/src/Plugin/PdfBackendInterface.php
   @@ -0,0 +1,96 @@
   +   * @return array[]
   +   *   An array of arrays containing metadata about the fields in the PDF. These
   +   *   can be iterated over and saved by the caller.
   +   *
   +   * @see \Drupal\fillpdf\Plugin\PdfBackendInterface::parseStream()
   +   */
   

   Noticed this while reviewing the function signature. Can we a bit more specific and perhaps show a sample array? It's keyed on field names, yeah?

   We could either introduce a value object that formalizes this (e.g. for those extra properties you wanted to use from PDFtk, when available)...or just document it. But as written, it's too hard for me to visualize what my plugin should be returning.

I want to run the FillPDF Service test locally one more time, and then after these tweaks, I think we're all good.

Reroll since we moved this to the fillpdf_legacy module.

Oh, and I think a small change like this is fine without tests. I don't really want to keep supporting the JavaBridge approach, anyway. FillPDF LocalServer has replaced it.