Initial baseline changes for IIIF auth 2.0

[masaball]

Related issue: #5239

TODO:

• Make auth probe its own endpoint instead of piggybacking on hls_manifest? Auth 2.0 probes are supposed to be GET requests, not HEAD requests so we would need to find another way to differentiate. May be easiest to use a separate method/endpoint.
• Make auth service changes in the playlist workflow (playlists_controller, iiif_playlist models, etc)
• Add auth 2 context info to top level manifest context
• Fix and add spec tests
• Find a iiif auth 2 client to test against and test against Ramp once it has been updated for iiif auth 2.
• Discussion on whether we want to make any changes to the headers and messages of the auth service.
• We use I18n translations for the headers and messages. Should we be using { "en": ["message"] } for these entries? It should be within spec to have the language marked "none", maybe that is better than specifying a specific language. Or maybe we can pull the default locale information?