DATACMNS-1126 - Add Kotlin constructor support.

[mp911de]

We now discover Kotlin constructors and apply parameter defaulting to allow construction of immutable value objects. Constructor discovery uses primary constructors by default and considers @PersistenceConstructor annotations.

ClassGeneratingEntityInstantiator can instantiate Kotlin classes with default parameters by resolving the synthetic constructor. Null values translate to parameter defaults.

class Person(val firstname: String = "Walter") {
}

class Address(val street: String, val city: String) {

	@PersistenceConstructor
	constructor(street: String = "Unknown", city: String = "Unknown", country: String) : this(street, city)
}

[odrotbohm]

Does it make sense to hide the dedicated constructor discoverer in BasicPersistentEntity's constructor? That avoids the first one to run in the first place. Looks like the already existing PreferredConstructorDiscoverer got a Kotlin code path as well now.

Also, I wonder If we can't have a dedicated Kotlin-aware EntityInstantiator to more cleanly separate that from the standard one. We should be able to detect that it's a Kotlin class we have to deal with and the just select the right EntityInstantiator in the first place, right?

[mp911de]

I'd turn PreferredConstructorDiscoverer into an interface with static factory methods and enums for the particular implementations (Java and Kotlin).

An EntityInstantiator for Kotlin classes is mostly the same as an instantiator for Java classes. Only Kotlin constructors with default arguments require a more specific instantiation. We could create a subclass ClassGeneratingKotlinEntityInstantiator extending ClassGeneratingEntityInstantiator and differentiate in EntityInstantiators which one to use.