Add a new interface in training service: querying capability/semantics of a training service

[QuanluZhang]

What would you like to be added: add a new interface in NNI training service. With this interface, users could query the capacity and the provided semantics of a certain training service. For example, when writing a tuner, users may want to know whether the training service provides job retry. They may also want to know the semantics of metric data report. Does it follow exactly once semantic, or the same metric data may be sent multiple times, or the order of the metric data is guaranteed or not.

Without this feature, how does current nni work：not clear

Components that may involve changes: training service