Move base64_image related functions to common directory (#10957)

### What problem does this PR solve?

As title

### Type of change

- [x] Refactoring

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Signed-off-by: Jin Hai <haijin.chn@gmail.com>

rag/flow/splitter/splitter.py

@@ -18,7 +18,7 @@ from functools import partial
 import trio
 
 from common.misc_utils import get_uuid
-from api.utils.base64_image import id2image, image2id
+from common.base64_image import id2image, image2id
 from deepdoc.parser.pdf_parser import RAGFlowPdfParser
 from rag.flow.base import ProcessBase, ProcessParamBase
 from rag.flow.splitter.schema import SplitterFromUpstream