Add a helper script to automate the TUF updates.

**DO NOT MERGE THIS**

If the code is ok after review we should integrate this work into the `bitmask_bundler` repository.
I'm making a pull request here since that repo is empty until we do some push.
The repo don't need to have the folder _metadata.staged_, I think we have it now, but better to fetch the folder _metadata_ and rename it as _metadata.staged_.

Soon there will be an unstable repo, this wget will need to be updated to fetch from unstable or stable depending which bundle you are generating.