feat: Implement static killswitches

[jan-auer]

This implements a first version of killswitches. Switches are configured through
objectstore config, so in order to create a killswitch one has to apply new
config and restart the service.

Note that all scope values must be strings. Examples for configuration look like this:

killswitches:
  # Drop all requests for a specific usecase, regardless of scopes
  - usecase: broken_usecase

  # Drop requests from a specific org, all projects, all usecases
  - scopes:
      org: "42"

  # Drop requests from a specific project, all usecases
  - scopes:
      org: "42"      # can also be omitted
      project: "4711"

  # Combination of usecase and org
  - usecase: attachments
    scopes:
      org: "42"

At a later stage, we can replace the static configuration with a runtime system.

Ref FS-227

[linear]

FS-227 Add killswitches