[jaxrs-spec] Add builders to models

Created by: shybovycha

As per #2415, adding builders generation for models.

The reasons for this change are:

1. currently the setters do not have the set prefix, which makes models not comply with java beans requirements
2. constructing models becomes painful, specifically when they have numerous fields

Compare the current way to construct a model to the proposed one:

now:

new Model().property1(value1).property2(value2)

// getters/setters

myModel.getProperty1();
myModel.property1(value1);

proposed:

Model.builder().property1(value1).property2(value2).build()

// getters/setters

myModel.getProperty1();
myModel.setProperty1(value1);

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