Import rag_tokenizer from Infinity (#11647)

### What problem does this PR solve?

- Original rag/nlp/rag_tokenizer.py is put to Infinity and infinity-sdk
via https://github.com/infiniflow/infinity/pull/3117 .
Import rag_tokenizer from infinity and inherit from
rag_tokenizer.RagTokenizer in new rag/nlp/rag_tokenizer.py.

- Bump infinity to 0.6.8

### Type of change
- [x] Refactoring

docker/infinity_conf.toml

@@ -1,5 +1,5 @@
 [general]
-version                  = "0.6.7"
+version                  = "0.6.8"
 time_zone                = "utc-8"
 
 [network]