feat: enable delete_blobs() to preserve generation

[cojenco]

This adds a flag preserve_generation to the method bucket.delete_blobs()

• allows preserving and propagating blob generations when set to True (default False)
• better ensures backwards compatibility with both delete_blobs() and bucket.delete(force=True)

Fixes #814

[andrewsg]

Looks good, but can we additionally update the docstring to mention that the default behavior is to drop the generation? That way preserve_generation makes sense in context.

[cojenco]

Looks good, but can we additionally update the docstring to mention that the default behavior is to drop the generation? That way preserve_generation makes sense in context.

Great idea, I've noted the default behavior in the docstring