Cleanup Context type alias

[AustinScola]

Is your feature request related to a problem?

When typing an application, there is no type alias for a cleanup context available for users to use.

Describe the solution you'd like

Add a type alias CleanupContext to aiohttp.typedefs:

CleanupContext = Callable[[Application], AsyncIterator[None]]

Describe alternatives you've considered

• Define it myself in my code.
• Don't bother typing it

Related component

Server

Additional context

This is similar to #5842 which was addressed in #5847.

Code of Conduct

• I agree to follow the aio-libs Code of Conduct

[Dreamsorcerer]

What's an example of when this would be used?

[AustinScola]

Here is an example of how a user might use this:

from typing import List

from aiohttp.web import Application

from server.contexts.database_client import database_client_context
from server.contexts.web_client import web_client_context

app = Application()

contexts: List[CleanupContext] = [database_client_context, web_client_context]
for context in contexts:
    app.cleanup_ctx.append(context)

[AustinScola]

It could also be used internally in aiohttp.

[Dreamsorcerer]

Seems reasonable enough. I've added it to #4686. Feel free to make a PR (also, one for Middleware if you fancy closing out that issue).

[AustinScola]

Will do.