authz/srp: add Enabled(), fix ActorPermissions()
Created by: bobheadxi
Combined from https://github.com/sourcegraph/sourcegraph/pull/27206 and https://github.com/sourcegraph/sourcegraph/pull/27189
- remove custom conf cache
- move check to checker interface - this makes it much easier to deal with in tests
- fix
ActorPermissions
- we should check for internal before unauthenticated