Allow for user-provided GCP bucket encryption key

[ericsmalling]

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When the GCS Bucket is encrypted with customer-managed key (which is what my client does), GCS list will not include the field md5Hash in the response. Therefore, they want to use the etag field instead of md5Hash.

┆Issue is synchronized with this Jira Story by Unito
┆Issue Number: MED-4

[c3-clement]

Indeed, when the GCS bucket is encrypted with customer-managed key, the List operation from the GCS API won't expose the md5Hash field, which cause Medusa code to crash there https://github.com/thelastpickle/cassandra-medusa/blob/master/medusa/storage/google_storage.py#L81 .

Note that this behavior is not documented anywhere, but I opened a ticket to GCP support.
See our discussion below

[c3-clement]

GCP team also have ticket about this opened since 2018:
https://issuetracker.google.com/issues/115290740#comment1

[ericsmalling]

Returning from a break, will be working on this asap