Add support for property-specific converters

[christophstrobl]

This PR will introduce common infrastructure that allows identifying and serving converters that are property specific. Other than existing converters configured via custom conversions, the property based variant will only be applied to a specific property of a type, instead of targeting all properties of a certain type.

PropertyValueConverter implementations will be provided a ValueConversionContext that gives access to the target property itself and may be enhanced by the store specific implementation.

class Person {
  // ...
  @ValueConverter(EncryptedValueConverter.class)
  String ssn;
}

class EncryptedValueConverter implements PropertyValueConverter<String, byte[], ValueConversionContext> {

  @Override
  public String read(byte[] value, ValueConversionContext context) {
    // ...
  }

  @Override
  public byte[] write(String value, ValueConversionContext context) {
    // ...
  }
}

In addition to the annotation based approach it is possible to programatically register converters for certain properties

PropertyValueConverterRegistrar registrar = new PropertyValueConverterRegistrar();

// untyped registration
registrar.registerConverter(Address.class, "street", new PropertyValueConverter() { ... });

// type safe registration
registrar.registerConverter(Person.class, Person::getSsn())
  .writing(value -> encrypt(value))
  .reading(value -> decrypt(value));

[mp911de]

Let's make the getPropertyValueConverter() method non-nullable as we have hasPropertyValueConverter(…) methods to check for the presence of converters.

[mp911de]

Having a null-object for PropertyValueConversions would allow to avoid nullable constructor args.

[mp911de]

Let's make the getPropertyValueConverter() method non-nullable as we have hasPropertyValueConverter(…) methods to check for the presence of converters.

[mp911de]

How about naming this method create? simple implies there could be also a complex variant.

[mp911de]

Didn't we want to have default methods or similar to avoid nullability for the value to read/to write?

[mp911de]

Let's make the getPropertyValueConverter() method non-nullable and introduce a hasConverter method to check for the presence of converters.

[mp911de]

We need to increase visibility so that builders can be used from outside of the package.

[mp911de]

Can we make the value mandatory?

[mp911de]

Let's make the getConverter() method non-nullable as we have containsConverterFor(…) methods to check for the presence of converters.

[mp911de]

Let's call this create as we have no other variants.

[mp911de]

Generics seem off here as T refers to the underlying type. Later on, we call registration.accept where generics do not match.

[mp911de]

That's merged, polished, and forward-ported now.