[META] Implement Validate Detector for AnomalyDetector Extension

[dbwiddis]

Is your feature request related to a problem?

Old plugin APIs for Validate Detector:

Validate the JSON format

POST _plugins/_anomaly_detection/detectors/_validate
POST _plugins/_anomaly_detection/detectors/_validate/detector

Validate the quality of the detector and get suggestions on improving

POST _plugins/_anomaly_detection/detectors/_validate/model

Components of this feature (Issues to be created and will be linked here):

For the detector validation:

• Create a new Rest Handler for the above routes - [FEATURE] Create a Rest Handler to add POST _validate/detector method #221
• Copy the detector validation logic from the existing plugin into the new handler - [FEATURE] Implement logic to validate detector type #222
• Process the response from the Rest Client into an appropriate ExtensionRestResponse - [FEATURE] Create validate detector type response #224
• Integrate the validation logic into the Create Detector (don't create a detector if it fails validation and provide useful error message) - [FEATURE] Integrate detector validation logic into create detector request #226

For the model validation:

• Copy the model validation logic from the existing plugin into the new handler - [FEATURE] Implement logic to validate detector model #223
• Process the response from the Rest Client into an appropriate ExtensionRestResponse - [FEATURE] Create validate detector model response #225

[dbwiddis]

Superceded by #353