Dev output compose

After recent big changes to the output handling in webui we were hitting an issue how to decide whether resulting datatype has a string::Compose overload. This could be handled only in webui but I think it is unnecessarily hard and unclean in current code. Also, the dance around serializing the graph when we were adding "invisible" string::Compose nodes was far from something optimal.

I was thinking and it is easily possible to run string::Compose always on the worker side and return the result in the webui (see previous commit). We can easily handle when the Compose overload exists and when it does not. We always return both the outputs (plain and pretty) and let the webui decide how to handle that.

Could we do the same for DotConverter?

cc @litavhan - what do you think about this solution?