**Description**

Support passing multiple prefixes to `GoogleCloudStorageListOperator` and `GoogleCloudStorageDeleteOperator` operators.

**Use case / motivation**

I have this folder structure in GCS bucket.

```
+-- year={year}
|   +-- month={month}
|       +--day={day}
|           +-- topic={topic1}
|                 +--file 1
|                 +--file 2
|                 +--file 3
|           +-- topic={topic2}
|                 +--file 1
|                 +--file 2
|                 +--file 3
|           +-- topic={topic3}
|                 +--file 1
|                 +--file 2
|                 +--file 3
|           +-- topic={topic4}
|                 +--file 1
|                 +--file 2
|                 +--file 3
|           +-- topic={topic5}
|                 +--file 1
|                 +--file 2
|                 +--file 3
|           +-- topic={topic6}
|                 +--file 1
|                 +--file 2
|                 +--file 3
|           +-- topic={topic7}
|       +--day={day}
|           +-- topic={topic1}
|                 +--file 1
|                 +--file 2
|                 +--file 3
|           +-- topic={topic2}
|                 +--file 1
|                 +--file 2
|                 +--file 3
|           +-- topic={topic3}
|                 +--file 1
|                 +--file 2
|                 +--file 3
|           +-- topic={topic4}
|                 +--file 1
|                 +--file 2
|                 +--file 3
|           +-- topic={topic5}
|                 +--file 1
|                 +--file 2
|                 +--file 3
|           +-- topic={topic6}
|                 +--file 1
|                 +--file 2
|                 +--file 3
|           +-- topic={topic7}
|                 +--file 1
|                 +--file 2
|                 +--file 3
|           ....
```

What I need to achieve is delete one day of objects. For example, I need to delete objects in `year=2020/month=08/day=19`. I can do that easily using `gsutils`. In `gsutil` you can delete them via wild card `gsutil ear=2020/month=08/day=19/*` but using the REST APIs you can't even if you use a prefix. The reason is there is no one prefix to get all the objects inside a folder. I achieved that by using multiple prefixes and for each prefix, I will get the list of objects. Unfortunately, I can't pass more than one prefix to the operators.

**Prefixes used**
- `year=2020/month=08/day=19/topic={topic1}`
- `year=2020/month=08/day=19/topic={topic2}`
- `year=2020/month=08/day=19/topic={topic3}`
- `year=2020/month=08/day=19/topic={topic4}`
- `year=2020/month=08/day=19/topic={topic5}`
- `year=2020/month=08/day=19/topic={topic6}`
- `year=2020/month=08/day=19/topic={topic7}`

"""Helper functions to upload a file or a string with a retry mechanism and exponential back-off.
            :param f: Callable that should be retried.
            """
            num_file_attempts = 0
            while num_file_attempts < num_max_attempts:
                try:
                    num_file_attempts += 1
                    f()
                except GoogleCloudError as e:
                    if num_file_attempts == num_max_attempts:
                        self.log.error(
                            "Upload attempt of object: %s from %s has failed. Attempt: %s, max %s.",
                            object_name,
                            object_name,
                            num_file_attempts,