This pull request contains a few changes:
1. Adds support for string ort values.
2. Fixes the training minimal build (that was broken with #16601) by
putting custom op registration behind #ifdefs
3. Fixes the iOS pod package generation (that was again broken with
#16601) by explicitly providing paths to be copied during pod creation.
Update Objective-C API to be more usable from Swift. E.g., to allow conversion from Objective-C methods with trailing NSError** parameter to throwing Swift methods.
Update CMake Objective-C framework setup.