[BUG] Add more tests coverage for AD transport layer

[Zhangxunmt]

The jacocoExclusions list includes lots of classes in AD transport layer. https://github.com/opensearch-project/anomaly-detection/blob/main/build.gradle#L513. They lack tests coverage which may cause bugs.
We should add tests into the following classes in the transport layer.

    'org.opensearch.ad.transport.StopDetectorRequest',
    'org.opensearch.ad.transport.StopDetectorResponse',
    'org.opensearch.ad.transport.CronRequest',

    // related to transport actions added for security
    'org.opensearch.ad.transport.DeleteAnomalyDetectorTransportAction*',
    'org.opensearch.ad.transport.GetAnomalyDetectorTransportAction*',
    'org.opensearch.ad.transport.SearchAnomalyResultTransportAction*',
    'org.opensearch.ad.transport.SearchAnomalyDetectorInfoTransportAction*',
    'org.opensearch.ad.transport.AnomalyDetectorJobRequest',

[Zhangxunmt]

The listed classes are removed from the jacoco exclusive list. PR: #498

[Zhangxunmt]

PR is merged. Closing this issue.