feat: add custom embedding types and migrate providers

- introduce baseembeddingsprovider and helper for embedding functions  
- add core embedding types and migrate providers, factory, and storage modules  
- remove unused type aliases and fix pydantic schema error  
- update providers with env var support and related fixes

src/crewai/rag/embeddings/providers/custom/embedding_callable.py

@@ -0,0 +1,22 @@
+"""Custom embedding function base implementation."""
+
+from crewai.rag.core.base_embeddings_callable import EmbeddingFunction
+from crewai.rag.core.types import Documents, Embeddings
+
+
+class CustomEmbeddingFunction(EmbeddingFunction[Documents]):
+    """Base class for custom embedding functions.
+
+    This provides a concrete implementation that can be subclassed for custom embeddings.
+    """
+
+    def __call__(self, input: Documents) -> Embeddings:
+        """Convert input documents to embeddings.
+
+        Args:
+            input: List of documents to embed.
+
+        Returns:
+            List of numpy arrays representing the embeddings.
+        """
+        raise NotImplementedError("Subclasses must implement __call__ method")