fix: make enum serializable with human-readable value (#555)

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

docling/datamodel/pipeline_options.py

@@ -126,6 +126,26 @@ class OcrMacOptions(OcrOptions):
     )
 
 
+# Define an enum for the backend options
+class PdfBackend(str, Enum):
+    """Enum of valid PDF backends."""
+
+    PYPDFIUM2 = "pypdfium2"
+    DLPARSE_V1 = "dlparse_v1"
+    DLPARSE_V2 = "dlparse_v2"
+
+
+# Define an enum for the ocr engines
+class OcrEngine(str, Enum):
+    """Enum of valid OCR engines."""
+
+    EASYOCR = "easyocr"
+    TESSERACT_CLI = "tesseract_cli"
+    TESSERACT = "tesseract"
+    OCRMAC = "ocrmac"
+    RAPIDOCR = "rapidocr"
+
+
 class PipelineOptions(BaseModel):
     """Base pipeline options."""