[FEATURE] Add specs for `knn` namespace

[nhtruong]

Part of Adding missing API specs issue

• /_opendistro/_knn/stats/: ["GET"]
• /_opendistro/_knn/stats/{stat}: ["GET"]
• /_opendistro/_knn/warmup/{index}: ["GET"]
• /_opendistro/_knn/{nodeId}/stats/: ["GET"]
• /_opendistro/_knn/{nodeId}/stats/{stat}: ["GET"]

[nhtruong]

@vamshin @navneet1v, Would you mind assigning this Issue to the appropriate person? Thank you.

[navneet1v]

@nhtruong based on this comment from @dblock do we still need to add api specs for these urls?

[dblock]

I should probably take that back, what's the correct way to add and mark these deprecated? And is there a way to provide aliases since these are identical to other _knn APIs?

[nhtruong]

Just responded #168 (comment)

@navneet1v, if these endpoints are identical to the _plugins counterparts (minus the fact they are deprecated), then our tools will handle it. If not, you'll have to redefine them. For this ticket, would you mind going over the _plugins/knn/ endpoints to make sure that they are correct?

[dblock]

For knn we're missing things like the responses from GET /_plugins/_knn/models/{model_id}. I was fixing a fluke in #644 and ran into this.

[nhtruong]

@vamshin are you still working on this item or is it done?

[dblock]

@nathaliellenaa was adding various knn routes