✨ Add support for shared/top-level parameters (dependencies, tags, etc) (#2434)

* ✨ Add Default and DefaultPlaceholder data structures

to handle defaults and overrides

* ✨ Add utils to get values by priority handling DefaultPlaceholders

* ✨ Add support for top-level parameters in FastAPI, APIRouter, include_router

including: prefix, tags, dependencies, deprecated, include_in_schema, responses, default_response_class, callbacks

* ♻️ Update openapi utils to handle DefaultPlaceholder for response_class

* 📝 Update bigger-application example code to use top-level params

and showcase them in APIRouter, FastAPI, include_router

* 📝 Update docs for Bigger Applications, include diagrams, top-level params

* 🔥 Simplify code and docs for callbacks as default_response_class is no longer required

* 📝 Add docs for top-level dependencies, in FastAPI()

* 📝 Add docs reference to top-level dependencies in docs for decorator

* ✅ Update/increase tests for Bigger Applications including shared parameters

* ✅ Add tests for top-level dependencies in FastAPI()

* ✅ Add tests for internal DefaultPlaceholder

* ✅ Update/increase tests for callbacks with top-level parameters

* ✅ Add LOTS of tests covering branches and cases for shared parameters

in top-level FastAPI, path operations, include_router, APIRouter, its path operations, nested include_router, nested APIRouter, and its path operations

* 🎨 Format/reorder parameters for consistency in FastAPI, APIRouter, include_router

docs_src/dependencies/tutorial012.py

@@ -0,0 +1,25 @@
+from fastapi import Depends, FastAPI, Header, HTTPException
+
+
+async def verify_token(x_token: str = Header(...)):
+    if x_token != "fake-super-secret-token":
+        raise HTTPException(status_code=400, detail="X-Token header invalid")
+
+
+async def verify_key(x_key: str = Header(...)):
+    if x_key != "fake-super-secret-key":
+        raise HTTPException(status_code=400, detail="X-Key header invalid")
+    return x_key
+
+
+app = FastAPI(dependencies=[Depends(verify_token), Depends(verify_key)])
+
+
+@app.get("/items/")
+async def read_items():
+    return [{"item": "Portal Gun"}, {"item": "Plumbus"}]
+
+
+@app.get("/users/")
+async def read_users():
+    return [{"username": "Rick"}, {"username": "Morty"}]