Issue #2922465 by wizonesolutions: Make backend plugins reusable

fillpdf.services.yml

src/Annotation/BackendService.php

src/FieldMapping.php

src/FieldMapping/ImageFieldMapping.php

src/FieldMapping/TextFieldMapping.php

src/FillPdfBackendPluginInterface.php

src/Plugin/BackendServiceBase.php

src/Plugin/BackendServiceInterface.php

src/Plugin/BackendServiceManager.php

src/Service/FillPdfContextManager.php

...s/modules/fillpdf_test/src/Plugin/BackendService/Test.php

...pdf_test/src/Plugin/FillPdfBackend/TestFillPdfBackend.php

tests/src/Functional/AdminIdTest.php

tests/src/Functional/FillPdfTestBase.php

tests/src/Functional/OverviewFormTest.php

tests/src/Functional/PdfPopulationTest.php

tests/src/Traits/TestFillPdfTrait.php

tests/src/Unit/FieldMapping/ImageFieldMappingTest.php

tests/src/Unit/FieldMapping/TextFieldMappingTest.php