Spike: PoC of secure-files-downloader within gitlab-cli

Goals:

• Set up secure-files-downloader to work locally from the command line
• Set up secure-files-downloader to work locally on a runner
• Create a working PoC of the secure-files-downloader that can be invoked with glab securefile download-all
• Investigate glab vs secure-files-downloader authentication, and what work would be required to integrate the two
• Investigate how to pass secure-files-downloader config values to glab, and what work would be required to integrate the two
• Determine any other unknowns not currently surfaced
• A reasonable sense of the remaining work, to be broken up into individual issues
• A PoC branch that can be built on for the actual implementation