Add --all flag to securefile download to download all securefiles of a project

To support Mobile DevOps CLI Integration Support Request (gitlab-org/incubation-engineering/mobile-devops/download-secure-files#44 - closed), we want to support an optional --all flag to download all of a project's secure files.

Discussion around the implementation can be found in this thread: !2333 (comment 2713896550)

--output-dir and --path flags should be mutually exclusive.