feat: Support getters/setters for annotations

[Christopher-Chianelli]

Is your feature request related to a problem? Please describe.
See TimefoldAI/timefold-solver-python#86

Describe the solution you'd like
The ability to annotate a getter/setter function pair for annotations that require setting fields, like PlanningVariable or PlanningEntityCollection:

class MyEntity:
    entities: list[Entity]
    
    def get_entities(self) -> Annotated[list[Entity], PlanningEntityCollection]:
        return self.entities
    
    def set_entities(self, entities):
        self.entities = entities

Describe alternatives you've considered
None

Additional context
Useful for whoever is migrating from optapy.

Note that Java will check for method names "get..." and "set...", so we will need to generate two additional methods. However, attributes also generate getter and setter methods, so it should has a prefix to distinguish it from the attribute getter/setter.