Janus aggregator API always reports being able to run either role

The configuration endpoint in the Janus aggregator API is hard-coded to report that it can execute either the leader or helper role (1). But that's not necessarily true: if someone exclusively runs a janus_aggregator but not the other three components, they can't actually act as a leader. Worse, they will accept report uploads and collection job requests, but the task will silently fail, because aggregation jobs will never be scheduled or run, and collection jobs will never be run.

We could add a configuration option allowing aggregators to opt out of either role. Then the aggregator API could report the corresponding supported roles, and the DAP API endpoints could return some HTTP client error if e.g. a helper-only Janus receives an upload request.

Start with aggregator_api/src/routes.rs at the configuration endpoint cited in the issue, then trace the DAP API endpoints that accept report uploads and collection job requests. Determine how role configuration should be represented and how unsupported roles should be reported or rejected. Done means the configuration reflects enabled roles and unsupported requests return a client error instead of silently failing.