Declarative REST clients

Enable configuration of REST clients in light modules, as YAML files, without requiring the Java "proxy" class

1. Offer versatile definitions taking e.g. host-names, supported operations map, system access credentials (optionally linking to an entry from password-manager), remote endpoint version
2. Support RO collection-fetching/querying operations, optionally with paging, so content can be displayed in a REST content app / browser, and aggregated in Periscope/find bar results.
3. Support single-item read operations & mapping to a Content Type model, so content can be presented in a detail sub-app.
4. Consider shifting REST & REST-client framework support to main/core.