Storage: add generation arguments (from #7023).

[tseaver]

• Uses Storage: ensure that 'Blob.reload' passes encryption headers. #7441 as a base.
• Merges in @jmcarp's work from Add missing generation arguments for bucket and blob methods. #7023, with my requested changes from that PR.
• Closes Add missing generation arguments for bucket and blob methods. #7023.
• Implements Storage: fetch specific object version #5546, Storage: add support for deleting specific blob generations #5781, storage: cannot access / delete versioned objects #2463, support versioning options when fetching cloud storage buckets and objects #2538.

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[tseaver]

I've created #7478 to track the flaky systest, which I believe is unrelated to the changes in this PR (I cannot reproduce locally).

[frankyn]

Adding one question. Overall LGTM.

[tseaver]

The flaky module tear-down for storage system tests is unrelated: I've reported it in #7505.

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😕 The bad news is that it appears that one or more commits were authored or co-authored by someone other than the pull request submitter. We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that here in the pull request.

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[747project]

Thank you for this! Is there a timeframe on when we can expect this to appear in a pypi release of google-cloud-storage?

[max-sixty]

FYI this broke some of our tests - potentially on us but posting at least for others.

Now if a process has a blob and it's overwritten, blob.exists() is False. It used to be true.

Specifically, this was our fixture:

@pytest.fixture()
def existing_blob_name(project, bucket, blob_name_expanded, data):
    blob = gcs.client(project).get_bucket(
        bucket).blob(blob_name_expanded)
    blob.upload_from_string(dill.dumps(data))
    assert blob.exists()
    yield blob.name
    blob.delete()

This was our test (for an internal wrapper method):

def test_set_overwrite(existing_blob_name, bucket, project):
    # could do this with a parameter but so small
    with pytest.raises(ValueError):
        gcs.set(existing_blob_name, 'abc', bucket=bucket,
                project=project, overwrite=False)

    gcs.set(existing_blob_name, 'abc', bucket=bucket,
            project=project, overwrite=True)

This failed on blob.delete(), since the newly written blob has a different generation