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/config/optional_imports/protocols.py

@@ -0,0 +1,14 @@
+"""Protocol definitions for RAG factory modules."""
+
+from typing import Protocol
+
+from crewai.rag.config.types import RagConfigType
+from crewai.rag.core.base_client import BaseClient
+
+
+class ChromaFactoryModule(Protocol):
+    """Protocol for ChromaDB factory module."""
+
+    def create_client(self, config: RagConfigType) -> BaseClient:
+        """Creates a ChromaDB client from configuration."""
+        ...