feat: recursive search in subapps for handler

[devamin]

Middleware application rate limit was not applied on sub application handlers, I added a recursive search for handlers of sub apps so that they get caught in middleware ratelimit as well.

[devamin]

@laurentS what do you think please ?

[laurentS]

Hi @devamin thanks for sending in this PR!

A couple of thoughts:

• mypy is complaining in CI, can you check what the problem is?
• does it seem like supporting submounted routes for starlette as well would be a good idea?
• are all recursive calls guaranteed to terminate? This is called on each request, so we should take care not to make this call too expensive :)