[Core] Generic sleep function

[l0lawrence]

For places where we asyncio.sleep within core, and in other SDK packages such as azure-cosmos or azure-communication-identity we should replace this with a generic sleep function that works with both asyncio and trio/other transports.

This will prevent us from having to import asyncio outside of core except in special cases.

[l0lawrence]

We have a generic sleep under transport but what about Lock etc, and should we make this more easily accessible?