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Similar to the Rest Handler created for Create Detector or Hello World Extension or GET method in #213, create an action implementing ExtensionRestHandler with routes for the validate detector API calls handled by appropriate methods with placeholder code:

POST /detectors/_validate
POST /detectors/_validate/{type}

Java code:

private ExtensionRestResponse handleValidateDetectorRequest(ExtensionRestRequest request) {
    // do things with request
    return new ExtensionRestResponse(request, OK, "placeholder");
}

Create a POST _validate Rest handler for validate detector similar to this. The business logic will be later done in #222.