Default to String for custom types that do not have a mapping

[martinbonnin]

Would be nice if DateTime for an example could by default be readable as a string for users that did not register any custom adapters.

[sav007]

@martinbonnin It's not so simple.

If we missing adapter but we have mapping configuration in apollo plugin:

customTypeMapping {
        DateTime = "java.util.Date"
    }

compiler generates models with the java.util.Date rather then String. But when we reading response field value of DateTime we are reading it as a String and then we will fail as we can't just assign String to java.util.Date and we don't have the way to convert it.

So the only solution is next: if we missing mapping in apollo plugin (build.gradle), string will be used by default as type in generated models for custom scalar types

[martinbonnin]

If there's no adapter, I don't expect anyone to put customTypeMapping in build.gradle. So this would really be a 'zero configuration' thing to make it easier to start using the project.

Agree with using a default return type if no mapping is defined. Maybe use 'Object' instead of 'String' so we could return directly what's in the json ?