Allow scitoken overrides in the cache #3027
Description
ONLY is the cache is unregistered, we should allow the administrator to specify a scitoken overrides file that gets merged into the calculated scitoken file.
This is analogous to what is done for the origin.
When adding the new configuration parameter, mark it as runtime reloadable. Make sure the file to use (if a file is used) can indeed be changed at runtime.
The goal is to have this be a mechanism where specific caches can continue to serve objects even when the origin is in downtime. Add an integration test along those lines.