NIFI-16362: Report Connector backlog support

[markap14]

Summary

• Expose supportsBacklogReporting on ConnectorDTO, matching the existing ProcessorDTO field from NIP-31.
• Populate the field in DtoFactory.createConnectorDto from whether the Connector implements BacklogReportingConnector, the same capability check used by Connector backlog REST handling.

https://issues.apache.org/jira/browse/NIFI-16362

Test plan

• Confirm a Connector that implements BacklogReportingConnector (for example KafkaToS3) returns supportsBacklogReporting: true in the Connector REST DTO.
• Confirm a Connector that does not implement backlog reporting returns supportsBacklogReporting: false.
• Confirm Processor backlog reporting and Connector backlog fetch/verify paths are unchanged.

[markap14]

[cursor-grok-4.6] macos-15 Java 21 (system-tests) failed on an unrelated clustering test; rerunning that shard.

Failed check: macos-15 Java 21 (run 35262989870)

org.apache.nifi.tests.system.clustering.OffloadIT.testOffload -- Time elapsed: 642.0 s <<< ERROR!
java.util.concurrent.TimeoutException: testOffload() timed out after 10 minutes
	at org.apache.nifi.tests.system.NiFiSystemIT.waitForNodeStatus(NiFiSystemIT.java:472)
	at org.apache.nifi.tests.system.clustering.OffloadIT.testIteration(OffloadIT.java:87)

Node log:

WARN [Offload FlowFiles from Node] org.apache.nifi.cluster.coordination.node.NodeClusterCoordinator Attempted to finish node offload for localhost:5672 but node is not in the offloading state, it is currently DISCONNECTED.

This PR only adds supportsBacklogReporting on Connector REST DTOs. It does not touch clustering or offload. The same system-tests run passed on ubuntu-24.04 Java 21/25 and macos-15 Java 25. Treating this as a pre-existing flake and rerunning the failed shard.