Add model management APIs

[ylwu-amzn]

After model trained, it will be stored in ML model index. But we don't have APIs to let user manage these models. In current design, the model is immutable once trained, so we don't need to provide update API. Such APIs should be added:

• Get model
• Delete model
• Search model

[ylwu-amzn]

#117 add ML Model get API
#136 add ML Delete model API
#140 add ML Model Search API