Let plugin modules register new webviz storage types

[HansKallekleiv]

Plugin modules should be able to store datatypes not covered by the default webviz_store return types defined here.
Examples are xtgeo classes used extensively in webviz-subsurface.
Currently we have quite a bit of overhead when converting these data to pd.DataFrame and io.BytesIO for storage.

Describe the solution you'd like

• Each return type allowed in webviz_store should be a separate class with a load and save function
• Plugin modules should be able to register new classes e.g. as entry_points.