Allow adding/getting metadata on the model level

Problem We would like to be able to store some types of metadata of the level of the model, e.g instance holdout splits. Currently there is no easy way to store metadata with the trained model, except if not through OpParams, stage metadata or manually if a separate file.

Solution We would like to add two methods to OpWorkflowModel class to allow saving/loading metadata with the model:

class OpWorkflowModel { 
   def addMetadata[T <: Product](name: String, value: T): this.type
   def getMetadata[T <: Product](name: String): Try[T]
}

Alternatives Store metadata in custom map in OpParams or in a separate file.

Additional context N/A