Serialization does not support custom port shapes

[Fadi-Albahrani]

I created a custom port shape that extends the PortShape,

When using the fromJson factory, it returns ArgumentError('Unknown port shape type: $type'), as it does not know about the custom port shape.

[pavanpodila]

this interface has changed to a MarkerShape, which is a more generalized shape that can work for both Connection endpoints and for Ports. There are some limited MarkerShapes right now that you can leverage. We will be expanding support for serialization on custom shapes in subsequent releases.