langchain[minor]: Add PebbloRetrievalQA chain with Identity & Semantic Enforcement support (#20641)

- **Description:** PebbloRetrievalQA chain introduces identity
enforcement using vector-db metadata filtering
- **Dependencies:** None
- **Issue:** None
- **Documentation:** Adding documentation for PebbloRetrievalQA chain in
a separate PR(https://github.com/langchain-ai/langchain/pull/20746)
- **Unit tests:** New unit-tests added

---------

Co-authored-by: Eugene Yurtsev <eugene@langchain.dev>

libs/community/langchain_community/chains/pebblo_retrieval/models.py

@@ -0,0 +1,62 @@
+"""Models for the PebbloRetrievalQA chain."""
+
+from typing import Any, List, Optional
+
+from langchain_core.pydantic_v1 import BaseModel
+
+
+class AuthContext(BaseModel):
+    """Class for an authorization context."""
+
+    name: Optional[str] = None
+    user_id: str
+    user_auth: List[str]
+    """List of user authorizations, which may include their User ID and 
+    the groups they are part of"""
+
+
+class SemanticEntities(BaseModel):
+    """Class for a semantic entity filter."""
+
+    deny: List[str]
+
+
+class SemanticTopics(BaseModel):
+    """Class for a semantic topic filter."""
+
+    deny: List[str]
+
+
+class SemanticContext(BaseModel):
+    """Class for a semantic context."""
+
+    pebblo_semantic_entities: Optional[SemanticEntities] = None
+    pebblo_semantic_topics: Optional[SemanticTopics] = None
+
+    def __init__(self, **data: Any) -> None:
+        super().__init__(**data)
+
+        # Validate semantic_context
+        if (
+            self.pebblo_semantic_entities is None
+            and self.pebblo_semantic_topics is None
+        ):
+            raise ValueError(
+                "semantic_context must contain 'pebblo_semantic_entities' or "
+                "'pebblo_semantic_topics'"
+            )
+
+
+class ChainInput(BaseModel):
+    """Input for PebbloRetrievalQA chain."""
+
+    query: str
+    auth_context: Optional[AuthContext] = None
+    semantic_context: Optional[SemanticContext] = None
+
+    def dict(self, **kwargs: Any) -> dict:
+        base_dict = super().dict(**kwargs)
+        # Keep auth_context and semantic_context as it is(Pydantic models)
+        base_dict["auth_context"] = self.auth_context
+        base_dict["semantic_context"] = self.semantic_context
+        return base_dict