Add jws-diag modcluster command: display mod_cluster listener configuration

[onkar717]

Summary

• Adds ModClusterParser — standalone DOM scan of server.xml for <Listener className="*ModCluster*"> elements; extracts known attributes and captures unknown ones in an extraAttributes map
• Adds ModClusterConfig value object with all known mod_cluster listener attributes and their documented defaults (connector, advertise, advertiseGroupAddress, advertisePort, proxyList, balancer, stickySession, stickySessionCookie)
• Adds modcluster sub-command with --format human|json output
• Human output: labeled key/value display with explicit vs default provenance; exits with code 2 when no mod_cluster listener is found
• JSON output: full config object with found: false sentinel when absent
• Zero changes to existing config model, parser, or command code

Design decisions

• Standalone DOM scan avoids coupling to the existing ServerXmlParser pipeline; the mod_cluster <Listener> is an infrastructure concern, not a per-<Service> concern
• extraAttributes map captures forward-compatible unknowns so operator-specific attrs are not silently dropped
• Defaults match the mod_cluster 1.3/2.x documented defaults so output is always complete even when attrs are omitted from server.xml

Test plan

• ModClusterParserTest full config fixture, defaults-only fixture, no listener fixture
• ModClusterHumanFormatterTestgolden snapshot for found / not-found cases
• ModClusterJsonFormatterTest golden snapshot for found / not-found cases
• mvn verify passes (391 tests, 0 failures)

[onkar717]

@dsoumis