Add support for in-memory View-based representations

[pcdavid]

It is possible to register View-based representation create programmatically or loaded from a .view model using IRepresentationDescriptionRegistryConfigurer, but the resulting representations can not actually be instanciated. This is because in Sirius Web, ViewRepresentationDescriptionSearchService only consults the documentRepository to resolve a respresentation sourceId, i.e. it only knows about View models which are actually stored in the database.

It should be possible to contribute in-memory View-based representations and use them even if they are never stored in the DB.