Add Serializable Vars State with nested fields descriptors

[nikolay-egorov]

[nikolay-egorov]

empty map should be sent in metadata

Maybe not empty, but just one-depth stringed values as it was before?

[ileasile]

I think that empty is better. If the client does not support this functionality, it will not benefit from any data we'll send in this map.

[nikolay-egorov]

Wandering through the park, I came across this. We could completely go for jvm fields instead of KProperties since there are some cases when we need jvms