Exposing a timeout parameter to run_async

## Description

`run_async` doesn't expose a timeout parameter, so users can't customize it without dropping down to the lower-level job API
Is there any reason not to add an optional timeout parameter to `run_async`:

```
def run_async(self, query, *, timeout=600., delete=True, ...):
    ...
    job.wait(timeout=timeout)
    ...
```

Would you want to open a PR for this?