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/core/base_client.py

@@ -3,6 +3,8 @@
 from abc import abstractmethod
 from typing import Any, Protocol, runtime_checkable, TypedDict, Annotated
 from typing_extensions import Unpack, Required
+from pydantic import GetCoreSchemaHandler
+from pydantic_core import CoreSchema, core_schema
 
 
 from crewai.rag.types import (
@@ -96,6 +98,17 @@ class BaseClient(Protocol):
     client: Any
     embedding_function: EmbeddingFunction
 
+    @classmethod
+    def __get_pydantic_core_schema__(
+        cls, _source_type: Any, _handler: GetCoreSchemaHandler
+    ) -> CoreSchema:
+        """Generate Pydantic core schema for BaseClient Protocol.
+
+        This allows the Protocol to be used in Pydantic models without
+        requiring arbitrary_types_allowed=True.
+        """
+        return core_schema.any_schema()
+
     @abstractmethod
     def create_collection(self, **kwargs: Unpack[BaseCollectionParams]) -> None:
         """Create a new collection/index in the vector database.