Add 'route' reference to Mount child scope

[y2kbugger]

Summary

When matching a mount point, record the Mount as scope["route"]

The purpose: when enriching logs with metadata, I would like to include the name of routes without having to re-traverse routes or rematch. Specifically, I'm focused on FastAPI endpoints and StaticFiles right now.

For fastapi endpoints, this is already easy:

"route_definition": "internalapp.endpoints.devtools.devtools:log_alternate_name"
"route_name": "my_custom_name"

But for handling StaticFiles mounts, this was the least intrusive way I could think to do it.
I'm open to other thoughts, especially if Mount is used more widely internally in a way that would make this a bad idea.

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.