[Core] Generic sleep function

For places where we `asyncio.sleep` within [core](https://github.com/Azure/azure-sdk-for-python/blob/db3b0e483968103a5142b5cc3a13f447b06bd582/sdk/core/corehttp/corehttp/transport/_base_async.py#L98), and in other SDK packages such as [azure-cosmos](https://github.com/Azure/azure-sdk-for-python/blob/db3b0e483968103a5142b5cc3a13f447b06bd582/sdk/cosmos/azure-cosmos/azure/cosmos/aio/_retry_utility_async.py#L193) or [azure-communication-identity](https://github.com/Azure/azure-sdk-for-python/blob/db3b0e483968103a5142b5cc3a13f447b06bd582/sdk/communication/azure-communication-identity/azure/communication/identity/_shared/utils_async.py#L25) 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.