Serialization: add CustomData and better support for integrating with extensions

[jeewonhh]

Adding CustomData to SerializationData to support the serialization of customized data for extensions.

[Maxxen]

Im curious, why do you want this?
Allowing extensions to push custom serialization is a little scary as you could end up breaking the storage format very easily if the extension makes backwards incompatible changes to what it serializes. Additionally we will probably remove support for custom serializer implementations in general and move e.g. the json serializer into core.

[Mytherin]

Thanks!