Reconsider `Actor.main` for v2

It could take care of `asyncio.run` for example, or it could be a decorator. There isn't much value in it now, compared to `async with Actor`.

A few mockups:

```py
from apify import Actor

@Actor.main
async def main():
  Actor.push_data({})
```
```py
from apify import Actor

async def main():
  Actor.push_data({})

Actor.run(main())
```
