Add include_router to Starlette and Router

[Lancetnik]

Summary

It's a PR connected with FastAPI nested lifespan bugfix.
So, there is a raw include_router implementation to inherit it by the FastAPI next step.
Also, it merge all nested by mount applications' events and lifespans to main application (#649).

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[Kludex]

That's really not what I mean. include_router doesn't make sense for Starlette.

We have this related PR: #1988.

[Lancetnik]

@Kludex did you talk about run Host routes lifespan calling? Or something else? Can you explain please?