BigQuery: make jobs awaitable

I know BigQuery jobs are asynchronous by default. However, I am struggling to make my datapipeline async end-to-end.

Looking at [this JS example](https://medium.com/skyshidigital/what-bigquery-is-and-isnt-d092fa173d53), I thought it would be the most Pythonic to make a BigQuery job awaitable. However, I can't get that to work in Python i.e. errors when `await client.query(query)`. Looking at the source code, I don't see which method [returns an awaitable object](https://www.python.org/dev/peps/pep-0492/).

I have little experience in writing async Python code and found [this example that wraps jobs in a `async def coroutine`](https://stackoverflow.com/questions/53065340/python-how-to-big-query-asynchronous-tasks).
```
class BQApi(object):
def __init__(self):
self.api = bigquery.Client.from_service_account_json(BQ_CONFIG["credentials"])

async def exec_query(self, query, **kwargs) -> bigquery.table.RowIterator:
job = self.api.query(query, **kwargs)
task = asyncio.create_task(self.coroutine_job(job))
return await task

@staticmethod
async def coroutine_job(job):
return job.result()
```

The [`google.api_core.operation.Operation`](https://googleapis.github.io/google-cloud-python/latest/core/operation.html) shows how to use `add_done_callback` to asynchronously wait for long-running operations. I have tried that, but the following yields `AttributeError: 'QueryJob' object has no attribute '_condition'` :

```
from concurrent.futures import ThreadPoolExecutor, as_completed
query1 = 'SELECT 1'
query2 = 'SELECT 2'

def my_callback(future):
result = future.result()

operations = [bq.query(query1), bq.query(query2)]
[operation.add_done_callback(my_callback) for operation in operations]
results2 = []
for future in as_completed(operations):
results2.append(list(future.result()))
```

Given that jobs are already asynchronous, would it make sense to add a method that returns an awaitable?

Or am I missing something and is there an Pythonic way to use the BigQuery client with the async/await pattern?