Add a static TYPE variable to java models

[RicoYao]

Example:

public class Board {
  public static final String TYPE = "board";
}

This can be useful when one needs a string-based representation of the model type.

[shashachu]

lgtm. i originally thought it might be nice to camel or snake case the string names, but probably not necessary.

[rahul-malik]

The value of this variable should mirror what we use polymorphicTypeIdentifier for in Objective-C.

You should use the typeIdentifier getter on SchemaObjectRoot here since we allow the type strings to vary if algebraicTypeIdentifier is specified (which is intended to override the value of type in schemas if needed)

If you look at the ObjC implementation of polymorphicTypeIdentifier you will see it uses this getter method in SchemaObjectRoot

public struct SchemaObjectRoot: Equatable {
   ....
    var typeIdentifier: String {
        return algebraicTypeIdentifier ?? name
    }
}

[RicoYao]

Thanks for the catch! Done.

Also ran make integration_test to update the samples.