[NX-450] Component Source API

JIRA: https://issues.sonatype.org/browse/NX-450
Do we want to consider unavailability of the remote connection vs. this source being put out-of-service in Nexus? Note this is just the Nx2 terminology, we don't necessarily have to provide both states here since putting a source out-of-service could involve removing it from the list of active sources...