Adds a factory function to create an initialised AsyncSubtensor object.

[thewhaleking]

Factory function for creating an already-initialised AsyncSubtensor object. Meaning you can do

import bittensor as bt
subtensor = await bt.async_subtensor()
await subtensor.get_subnets(...

rather than the traditional

from bittensor.core.async_subtensor import AsyncSubtensor
subtensor = AsyncSubtensor()
async with subtensor:
    await subtensor.get_subnets(...