[FEATURE] Add specs for `anomaly_detection` namespace

[nhtruong]

Part of Adding missing API specs issue

• [ ] /_plugins/_anomaly_detection/detectors: ["POST"]
• [ ] /_plugins/_anomaly_detection/detectors/_preview: ["POST"]
• [ ] /_plugins/_anomaly_detection/detectors/_search: ["GET","POST"]
• [ ] /_plugins/_anomaly_detection/detectors/_validate: ["POST"]
• [ ] /_plugins/_anomaly_detection/detectors/_validate/{type}: ["POST"]
• [ ] /_plugins/_anomaly_detection/detectors/count: ["GET"]
• [ ] /_plugins/_anomaly_detection/detectors/match: ["GET"]
• [ ] /_plugins/_anomaly_detection/detectors/results: ["DELETE"]
• [ ] /_plugins/_anomaly_detection/detectors/results/_search: ["GET","POST"]
• [ ] /_plugins/_anomaly_detection/detectors/results/_search/{resultIndex}: ["GET","POST"]
• [ ] /_plugins/_anomaly_detection/detectors/tasks/_search: ["GET","POST"]
• [ ] /_plugins/_anomaly_detection/detectors/{detectorID}: ["GET","PUT","DELETE","HEAD"]
• [ ] /_plugins/_anomaly_detection/detectors/{detectorID}/_preview: ["POST"]
• [ ] /_plugins/_anomaly_detection/detectors/{detectorID}/_profile: ["GET","POST"]
• [ ] /_plugins/_anomaly_detection/detectors/{detectorID}/_profile/{type}: ["GET","POST"]
• [ ] /_plugins/_anomaly_detection/detectors/{detectorID}/_run: ["POST"]
• [ ] /_plugins/_anomaly_detection/detectors/{detectorID}/_start: ["POST"]
• [ ] /_plugins/_anomaly_detection/detectors/{detectorID}/_stop: ["POST"]
• [ ] /_plugins/_anomaly_detection/detectors/{detectorID}/results/_topAnomalies: ["GET","POST"]
• [ ] /_plugins/_anomaly_detection/stats/: ["GET"]
• [ ] /_plugins/_anomaly_detection/stats/{stat}: ["GET"]
• [ ] /_plugins/_anomaly_detection/{nodeId}/stats/: ["GET"]
• [ ] /_plugins/_anomaly_detection/{nodeId}/stats/{stat}: ["GET"]

[kaituo]

@minalsha Can you also take a look ? The client team wants to get it done before end of July.

[kaituo]

Example: https://github.com/opensearch-project/opensearch-api-specification/blob/main/spec/namespaces/_core.yaml#L30-L76

Raising PR(s) is the way to go.