"Machine Learning Essentials" toolkits need model metadata

[syoutsey]

These toolkits need metadata toolkit type associated with the exported Core ML model (Boosted Trees, Logistic Classifier, etc.). Related: #2794.

For example, the Style Transfer toolkit exposes the type key as part of the Creator Defined Core ML keys:

creatorDefinedKey: {
    "com.github.apple.os.platform" = "Darwin-19.0.0-x86_64-i386-64bit";
    "com.github.apple.turicreate.version" = "6.0rc1";
    "content_feature" = image;
    "max_iterations" = 2;
    model = "resnet-16";
    "num_styles" = 1;
    "style_feature" = image;
    "training_iterations" = 2;
    type = "style_transfer";
    version = 1;
}