✨ Add support for not needing ... as default value in required Query(), Path(), Header(), etc. (#4906)

* ✨ Do not require default value in Query(), Path(), Header(), etc

* 📝 Update source examples for docs with default and required values

* ✅ Update tests with new default values and not required Ellipsis

* 📝 Update docs for Query params and update info about default value, required, Ellipsis

docs_src/header_params/tutorial001.py

@@ -6,5 +6,5 @@ app = FastAPI()
 
 
 @app.get("/items/")
-async def read_items(user_agent: Optional[str] = Header(None)):
+async def read_items(user_agent: Optional[str] = Header(default=None)):
     return {"User-Agent": user_agent}