Add means to retrieve responses after calling batch module

[shunping]

Problem:
The current Batch class (https://github.com/googleapis/python-storage/blob/v2.14.0/google/cloud/storage/batch.py#L145) does not provide a good way of getting the responses from the batch call.

• There is a private member variable _responses, but it is not exposed from any public API for straightforward usage.
• There is a public function named finish(), which returns the responses. However, it is called in the__exit__(), and without a hack (mentioned in [Task]: Update the minor version of google-cloud-storage library prior to Beam release. apache/beam#27326 (comment)), it is not really convenient to use.

Suggestion:
Add a simple public function to return _responses. It can be as simple as

def get_responses(self):
    return self._responses