onnxruntime

Objective-C binding for ORT training  (#16127)

### Description
Implement Objective-C binding for `ORTCheckPoint`. Additionally, 
- Modify `onnxruntime_objectivec.cmake` to only include training header
and sources when training flag is enabled
- Enable objective-c binding for `orttraining-mac-ci-pipeline`

### Motivation and Context
This PR is part of implementing Objective-C bindings for training API.
It implements objective-c binding for ORTCheckPoint class. The
objective-C API closely resembles the C++ API.

**Note**: The test for saving checkpoint is skipped as it requires use
of training session. It will be added when the objective-c binding for
`ORTTrainingSession` is added.