=head1 DESCRIPTION

Defines a type of object to serialize/deserialize from/to the 
JSON:API format.

=head1 ATTRIBUTES

=head2 registry

L<Dancer2::Plugin::JsonApi::Registry> to use to find the definition of
other object types.

=head2 before_serialize

Accepts a function, which will be called on the original C<$data> to serialize
to groom it.

    before_serialize => sub($data,$xtra) {
       # lowercase all keys
       return +{ pairmap { lc($a) => $b } %$data } 
    }

=head2 type

The JSON:API object type. Read-only, required.

=head2 id

Key to use as a reference to the object. Can be a string,
or a function that will be passed the original data object.
Read-only, defaults to the string C<id>. 

=head2 links 

Links to include as part of the object. 

=head2 top_level_links

Links to include to the serialized top level, if the top level object 
is of the type defined by this class.

=head2 top_level_meta

Meta information to include to the serialized top level, if the top level object 
is of the type defined by this class.

=head2 relationships

Relationships for the object type.

=head2 allowed_attributes

List of attributes that can be serialized/deserialized.

=head1 METHODS

=head2 top_level_serialize($data,$extra_data = {})

Serializes C<$data> as a top-level JSON:API object. 

=head2 serialize_data($data,$extra_data)

Serializes the inner C<$data>.