Allowing a different Constructor as the one used by ObjectBox

When I am creating Entities which have custom types, ObjectBox tries to use the default constructor or the first with parameter names matching the Entity's property names (https://docs.objectbox.io/entity-annotations#objectbox-database-persistence-with-entity-annotations).

If I understood correctly, this would enforce the creation of a custom constructor for use by the user while the default would be reserved for ObjectBox.

For example, the following does not build using the generator:

@Entity()
final class Person {
  Person.ob({
    required this.uid,
    required this.name,
    required this.id,
    required this.metadataBytes,
  }) : metadata = PersonMetadata.fromBytes(metadataBytes);

  Person({
    required this.uid,
    required this.name,
    required this.metadata,
    this.id = 0,
  });

  int id;

  @Index()
  final String uid;
  final String name;

  @Transient()
  final PersonMetadata metadata;

  @Property(type: PropertyType.byteVector)
  final ByteData metadataBytes;
}

class PersonMetadata {
  PersonMetadata.fromBytes(ByteData data);
  ByteData toBytes() => ByteData(0);
}

as ObjectBox is using the default Person constructor to deserialize the object.

Describe the solution you'd like

I though of four possible ways this could possibly be achieved:

• Addition of a new annotation to annotate the constructor to be used by the user to directly annotate the constructor that ObjectBox should use.
• Addition of a constructor field to the Entity annotation. If I am not mistaken, this is the approach used by JsonSerializable (https://github.com/google/json_serializable.dart/blob/85e83641befab6a30526c24950ecfd3fe3f9ce62/json_annotation/lib/src/json_serializable.dart#L66)
• Use the first constructor that has the required properties, independently from if it is the default. Meaning that in the example above, ObjectBox would use the .ob constructor even if a default is available, since the first one has all the required properties.
• Add the possibility of Adding the Entity decorator to the constructor itself (maybe this could be too confusing for a first user or to difficult to implement on the generator)

Start by reproducing the constructor-selection failure with the Person example and reviewing the generator behavior described in the issue. Compare the four proposed approaches, then confirm the chosen design supports a separate user constructor and ObjectBox deserialization without breaking existing entities.