Avoid multiple serialize-deserialize cycles when returning results to Language Server clients

[hubertp]

Every time Language Server returns a value reported by runtime we go through serialize-deserialize conversion.
Currently, this is necessary because runner and runtime jars are not loaded using the same classloader.

A proposal was made in #5419 to switch to in-memory protocol but that might involve a rather deep rewrite of Language Server.
This ticket proposes to consider introducing data structures that can be shared between the two process without the costly serialization.
We have to investigate if there is a way to workaround the classloader issue.