Feat: dataflow supports text (#10058)

### What problem does this PR solve?

dataflow supports text.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

rag/flow/tests/dsl_examples/general_pdf_all.json

@@ -44,9 +44,12 @@
                     "markdown"
                   ],
                   "output_format": "json"
+                },
+                "text": {
+                  "suffix": ["txt"],
+                  "output_format": "json"
                 }
               }
-            }
           }
         },
         "downstream": ["Chunker:0"],