feat: rag configuration with optional dependency support (#3394)

### RAG Config System

* Added ChromaDB client creation via config with sensible defaults
* Introduced optional imports and shared RAG config utilities/schema
* Enabled embedding function support with ChromaDB provider integration
* Refactored configs for immutability and stronger type safety
* Removed unused code and expanded test coverage

src/crewai/rag/__init__.py

@@ -1 +1,58 @@
-"""RAG (Retrieval-Augmented Generation) infrastructure for CrewAI."""
+"""RAG (Retrieval-Augmented Generation) infrastructure for CrewAI."""
+
+import sys
+import importlib
+from types import ModuleType
+from typing import Any
+
+from crewai.rag.config.types import RagConfigType
+from crewai.rag.config.utils import set_rag_config
+
+
+_module_path = __path__
+_module_file = __file__
+
+class _RagModule(ModuleType):
+    """Module wrapper to intercept attribute setting for config."""
+
+    __path__ = _module_path
+    __file__ = _module_file
+
+    def __init__(self, module_name: str):
+        """Initialize the module wrapper.
+
+        Args:
+            module_name: Name of the module.
+        """
+        super().__init__(module_name)
+
+    def __setattr__(self, name: str, value: RagConfigType) -> None:
+        """Set module attributes.
+
+        Args:
+            name: Attribute name.
+            value: Attribute value.
+        """
+        if name == "config":
+            return set_rag_config(value)
+        raise AttributeError(f"Setting attribute '{name}' is not allowed.")
+
+    def __getattr__(self, name: str) -> Any:
+        """Get module attributes.
+
+        Args:
+            name: Attribute name.
+
+        Returns:
+            The requested attribute.
+
+        Raises:
+            AttributeError: If attribute doesn't exist.
+        """
+        try:
+            return importlib.import_module(f"{self.__name__}.{name}")
+        except ImportError:
+            raise AttributeError(f"module '{self.__name__}' has no attribute '{name}'")
+
+
+sys.modules[__name__] = _RagModule(__name__)