Example: Add auth using global dependencies (#360)

Add an example that shows how to use FastAPI global dependencies for
auth.

Will follow up with more involved examples.

examples/auth/global_deps/server.py

@@ -0,0 +1,53 @@
+#!/usr/bin/env python
+"""An example that uses Fast API global dependencies.
+
+This approach can be used if the same authentication logic can be used
+for all endpoints in the application.
+
+This may be a reasonable approach for simple applications.
+
+See:
+
+* https://fastapi.tiangolo.com/tutorial/dependencies/global-dependencies/
+* https://fastapi.tiangolo.com/tutorial/dependencies/
+* https://fastapi.tiangolo.com/tutorial/security/
+"""
+
+from fastapi import Depends, FastAPI, Header, HTTPException
+from langchain_core.runnables import RunnableLambda
+from typing_extensions import Annotated
+
+from langserve import add_routes
+
+
+async def verify_token(x_token: Annotated[str, Header()]) -> None:
+    """Verify the token is valid."""
+    # Replace this with your actual authentication logic
+    if x_token != "secret-token":
+        raise HTTPException(status_code=400, detail="X-Token header invalid")
+
+
+app = FastAPI(
+    title="LangChain Server",
+    version="1.0",
+    dependencies=[Depends(verify_token)],
+)
+
+
+def add_one(x: int) -> int:
+    """Add one to an integer."""
+    return x + 1
+
+
+chain = RunnableLambda(add_one)
+
+
+add_routes(
+    app,
+    chain,
+)
+
+if __name__ == "__main__":
+    import uvicorn
+
+    uvicorn.run(app, host="localhost", port=8000)