[Story] Agent dual-registration

[andrewazores]

Currently, the Agent implements a Cryostat Discovery Plugin by creating a plugin registration (and corresponding REALM node), and defines a single target JVM node within that Realm, representing itself. This will normally have an Agent HTTP connectUrl, unless the config property for prefer-jmx is set, in which case the Agent will publish a JMX ServiceURL for itself if the JVM it's attached to appears to have JMX enabled.

Why not publish both URLs? The Agent HTTP URL will always be available and publish-able as one target JVM node. After #163 this URL may also support write/mutation operations. A simple check endpoint can be added to return that status so that the available features can be queried without first trying to perform a write operation and seeing if it fails. A second target JVM node under the same Realm could be optionally published as well, if the host JVM appears to have JMX enabled. Then the prefer-jmx config can be dropped.