Feat: Redesign and refactor agent module (#9113)

### What problem does this PR solve?

#9082 #6365

<u> **WARNING: it's not compatible with the older version of `Agent`
module, which means that `Agent` from older versions can not work
anymore.**</u>

### Type of change

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

rag/prompts/__init__.py

@@ -0,0 +1,6 @@
+from . import prompts
+
+__all__ = [name for name in dir(prompts)
+           if not name.startswith('_')]
+
+globals().update({name: getattr(prompts, name) for name in __all__})