Implement targets API proxy in the console backend

Thanos now exposes a federated Targets API (see https://github.com/thanos-io/thanos/pull/3350). The API needs to be proxied by the console backend to include targets information in the OCP admin console.

AC

• The console backend exposes an API endpoint to retrieve targets from the Thanos API via the OAuth proxy port (similar to what's done for query & query_range endpoints).