[META] Implement Get Detector for AnomalyDetector Extension

[owaiskazi19]

Is your feature request related to a problem?

Since we have the first feature up here for creating a detector.
Old plugin APIs for Get Detector:

1. GET _plugins/_anomaly_detection/detectors/<detectorId>
2. GET _plugins/_anomaly_detection/detectors/<detectorId>?job=true

This issue addresses the 1st request as 2nd has dependency on Job Scheduler. Until we finish the integration of Job Scheduler with AD Extension we can't start working on 2nd.

The proposed API for Get Detector with extensibility:

GET _extensions/_anomaly_detection/detectors/<detectorId>

The next milestone is to implement an API to get the detector which has already been created. The feature can be broken down as below:

1. Currently the detector id is not randomized and is a pure string in the response of create detector - [FEATURE] Randomized detector id for create/get detector #212
2. Create a Rest Handler to add GET method for the detector. - [FEATURE] Create a Rest Handler to add GET method for the detector #213
3. Implement the logic for searching the detector. - [FEATURE] Implement the logic for searching the detector #214
4. Create the response for Get Detector similar to old plugin architecture response. - [FEATURE] Create Get detector response #215

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[dbwiddis]

How are we tracking the job scheduler one? Separate meta issue for job scheduler integration, or should we list it here knowing we can't do it yet?

[owaiskazi19]

How are we tracking the job scheduler one? Separate meta issue for job scheduler integration, or should we list it here knowing we can't do it yet?

That's a good point. I am thinking of creating an issue and adding such features which are dependent on Job Scheduler there. We can link this issue with the meta issue which @joshpalis will create for Job Scheduler.
Update: Created #216

[owaiskazi19]

Covered this issue in #373 and #451