feat: gsutil file format mutator for same bucket

Hi Folks,

looking for the feature for allowing changing file formats in the same bucket through command line interface.

The bucket can have 1000's of files having same format for example:

- "application/octet-stream" which can be morphed to another format like "application/x-log".
- Command should also allow changing the file formats based on stored format field and not only the extension of the file.
- Command should work on the same bucket. There can be a temp bucket just for moving from one format and getting in another format.
- Lastly, the command line should also provide a list of the files that got changed.

Thanks!

No files, tests, or entry points are named. Start by locating gsutil's command-line handling for bucket and object metadata operations, then determine how same-bucket format changes, stored-format filtering, temporary movement, and reporting changed files would fit together. Done means a defined command behavior covering all requested cases, with verification for the affected files.

Written by the indexing model from the issue text.