[Feature] user custom instantiators

[frecco75]

The idea of this feature is to provide a mechanism for users to define custom instantiators like it exists for custom field value changers. It follows the same idea to let user extends pojo-tester mechanism.

It already exists a mechanism that let the user to select the choosen constructor (explanations here) but it is not efficient when the pojo use provided types from external libraries.

Here are the steps to add pojo tester support for an external librairy types (for example vavr) in an external project :

1. Step 1 : define instantiator for custom types (vavr types for example). You have to inherits the UserObjectInstantiator class
2. Step 2 : attach the custom instantiator :
   Instantiator.INSTANCE.attach(VavrTypesInstantiator.class);

Now we can use pojo tester with an object using vavr types, for example :

class User {
    private int id;
    private Option<String> email;  // Option is a Vavr type
}

Detailed example here with vavr collections.