[Task] Discovery plugin updates should require all branches terminate in leaf nodes

[andrewazores]

https://github.com/cryostatio/cryostat/blob/d6a438e721a3847bf55bcb3ee25bc117c0804a1b/src/main/java/io/cryostat/net/web/http/api/v2/DiscoveryPostHandler.java#L129

Plugin realm subtrees should always be published as a list of leaf (target) nodes, or as a list of environment nodes whose children are either more environment nodes or target nodes. All paths from the realm (the subtree root) downward should terminate in target nodes. Any branch that terminates (node with no children) in an environment node is a leafless branch and must be disallowed.