Create GET _cat/ml/trained_models API

[benwtrent]

See #51364 for more info on how to do this.

This API should return basic statistics and settings that effect those statistics.

I think the GetTrainedModelsStatsAction is a good place to start.

I do think that returning estimated model size + estimated num operations would be good information as well.

[elasticmachine]

Pinging @elastic/ml-core (:ml)