Allow shallow encoding a class

[SchoofsEbert]

When encoding a custom class, you don't always want to go all the way down to the primitives.
Sometimes you want to convert the object to a Map, e.g. to dynamically access field values for forms for example, but want to keep the types of the attributes when extracting them.

This PR generates a new method toShallowMap that converts the object to a map, but not the values of its attributes.

A new test case has been added to verify the correctness of the implementation.

If this does not follow the general idea of dart_mappable, is there a way we can implement this outside of the package?

[schultek]

Hi. I would be find with adding this to the EncodingOptions as you did, but not to the generated Mappable mixins. So then to use it you need to call return MyClassMapper.ensureInitialized() .encodeMap<MyClass>(myClass, EncodingOptions(shallow: true)); manually.

If that works for you I'm happy to merge this pr once its adjusted.

[SchoofsEbert]

The generation of the toShallowMap methods in the mixins has been removed.

[SchoofsEbert]

@schultek Do you have any other requirements before the PR can be merged?

[schultek]

Sry did't had time to check yet. LGTM