✨ Add support for BackgroundTasks parameters (#103)

* ✨ Add support for BackgroundTasks parameters

* 🐛 Fix type declaration in dependencies

* 🐛 Fix coverage of util in tests

docs/tutorial/background-tasks.md

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+You can define background tasks to be run *after* returning a response.
+
+This is useful for operations that need to happen after a request, but that the client doesn't really have to be waiting for the operation to complete before receiving his response.
+
+This includes, for example:
+
+* Email notifications sent after performing an action:
+    * As connecting to an email server and sending an email tends to be "slow" (several seconds), you can return the response right away and send the email notification in the background.
+* Processing data:
+    * For example, let's say you receive a file that must go through a slow process, you can return a response of "Accepted" (HTTP 202) and process it in the background.
+
+## Using `BackgroundTasks`
+
+First, import `BackgroundTasks` and define a parameter in your *path operation function* with a type declaration of `BackgroundTasks`:
+
+```Python hl_lines="1 13"
+{!./src/background_tasks/tutorial001.py!}
+```
+
+**FastAPI** will create the object of type `BackgroundTasks` for you and pass it as that parameter.
+
+!!! tip
+    You declare a parameter of `BackgroundTasks` and use it in a very similar way as to when <a href="/tutorial/using-request-directly/" target="_blank">using the `Request` directly</a>.
+
+
+## Create a task function
+
+Create a function to be run as the background task.
+
+It is just a standard function that can receive parameters.
+
+It can be an `async def` or normal `def` function, **FastAPI** will know how to handle it correctly.
+
+In this case, the task function will write to a file (simulating sending an email). 
+
+And as the write operation doesn't use `async` and `await`, we define the function with normal `def`:
+
+```Python hl_lines="6 7 8 9"
+{!./src/background_tasks/tutorial001.py!}
+```
+
+## Add the background task
+
+Inside of your *path operation function*, pass your task function to the *background tasks* object with the method `.add_task()`:
+
+```Python hl_lines="14"
+{!./src/background_tasks/tutorial001.py!}
+```
+
+`.add_task()` receives as arguments:
+
+* A task function to be run in the background (`write_notification`).
+* Any sequence of arguments that should be passed to the task function in order (`email`).
+* Any keyword arguments that should be passed to the task function (`message="some notification"`).
+
+## Dependency Injection
+
+Using `BackgroundTasks` also works with the dependency injection system, you can declare a parameter of type `BackgroundTasks` at multiple levels: in a *path operation function*, in a dependency (dependable), in a sub-dependency, etc.
+
+**FastAPI** knows what to do in each case and how to re-use the same object, so that all the background tasks are merged together and are run in the background afterwards:
+
+```Python hl_lines="11 14 20 23"
+{!./src/background_tasks/tutorial002.py!}
+```
+
+In this example, the messages will be written to the `log.txt` file *after* the response is sent.
+
+If there was a query in the request, it will be written to the log in a background task.
+
+And then another background task generated at the *path operation function* will write a message using the `email` path parameter.
+
+## Technical Details
+
+The class `BackgroundTasks` comes directly from <a href="https://www.starlette.io/background/" target="_blank">`starlette.background`</a>.
+
+It is imported/included directly into FastAPI so that you can import it from `fastapi` and avoid accidentally importing the alternative `BackgroundTask` (without the `s` at the end) from `starlette.background`.
+
+By only using `BackgroundTasks` (and not `BackgroundTask`), it's then possible to use it as a *path operation function* parameter and have **FastAPI** handle the rest for you, just like when using the `Request` object directly.
+
+It's still possible to use `BackgroundTask` alone in FastAPI, but you have to create the object in your code and return a Starlette `Response` including it.
+
+You can see more details in <a href="https://www.starlette.io/background/" target="_blank">Starlette's official docs for Background Tasks</a>.
+
+## Recap
+
+Import and use `BackgroundTasks` with parameters in *path operation functions* and dependencies to add background tasks.