NEXUS-4062: Signaling for Realm configuration states.

Core support and events that makes possible any realm to
signal is made "ready" (configured) or is made "not-ready"
(existing working configuration removed).

Core will based on these events activate (move to Selected Realms
from Available realms) the realm automatically, or deactivate
the realm (the opposite).

Issue
https://issues.sonatype.org/browse/NEXUS-4062

CI
https://bamboo.zion.sonatype.com/browse/NX-OSSF27