WIP: Schema Validation in client

[MichaelHirn]

As of right now, rlay (server) and the rlay-ontology does support any functionality to restrict the schema of an ontology. For example: Only http:Connection can have ObjectPropertyAssertion of type http:requests. W3C provides SHACL for this use-case and GraphQL has it's own schema definition language.

But, rlay-client-lib with it's generated client supports some very basic way of setting fields and defaults, which are used when .assert and .create are called. It seems that this could be a good start to get schema validation sorted, at least for the client. The functionality could well be provided by a third-party library (research and dd has not been untertaken so far) and could then be implemented very quickly.

However, so far support for schema validation has not been a blocker in any way and it seems like a nice-to-have further down the road. So this, issue is just for documenting with lowest priority.