Generate configuration keys metadata for immutable POJOs

[snicoll]

Follow up of #8762

We need to generate proper metadata for the immutable style as the rules are different. We currently check for the presence of a setter to determine what needs to be exposed. In the immutable case, constructor arguments will be used for this.

[snicoll]

Interesting to look at this from a metadata angle. I was wondering how we could infer default values. In a getter/setter arrangement, if you define a value with a primitive type, it gets its default value unless specified otherwise (i.e. a boolean flag is false by default).

With the constructor arrangement, it gets a tad more complex. If we ignore validation (@NotNull) we could probably infer false as well if the constructor parameter uses the primitive type. There is a test failing in the branch that illustrates that scenario.

[snicoll]

The metadata on the Java side has been improved to detect immutable types. When a @DefaultVaue is set, we try to coerce for primitive types and generate a better representation of it in the metadata.

Kotlin types are not supported as kapt still exposes several limitations. I've created #16293 to follow-up and try to improve this as much as we can.