HotReload should be an ASGI app

[florimondmanca]

HotReload should be an actual ASGI app that we can mount onto an app, instead of having to manually register its WebSocketEndpoint:

import arel
from starlette.applications import Starlette
from starlette.routing import Mount

hotreload = arel.HotReload("./path/to/files")

app = Starlette(
    routes=[Mount("/hot-reload", hotreload, name="hot-reload")],
    on_startup=[hotreload.startup],
    on_shutdown=[hot_reaload.shutdown],
)

Then the WebSocketEndpoint could be provided internally at /, so that the route name stays hot-reload, i.e. whatever is given to Mount.

Wouldn't remove the need to register event handlers (most frameworks don't support lifespan on sub-apps, eg see encode/starlette#649), but at least it's a bit less framework-specific surface.