Add "features" or "capabilities" support

[firstone]

The idea is that, both, polyglot server / UI and interface, and polyglot interface, could enable some enhanced functionality. So, for instance, if we implement enhances custom parameters, both, server and client need to support it. Both can define "feature" called "enhancedParameters". If, and only if, both have it defined, it would be enabled at runtime. If one side is missing it, it would fall back to the existing functionality. Then you could make potentially breaking changes without having to worry about breaking existing clients.

Further, it could be used to temporarily disable new functionality if it causes issues. You just have to disable "feature" on server or a client.