Model Builder: Show datacube/metadata information for return values

[m-mohr]

We should evaluate the process graphs in the model builder on the fly and show for the return values (i.e. when hovering over them) how the datacube looks like.

It needs either a way to describe datacube changes schematically in the process metadata or I'd need to implement "stubs" of the processes in the Web Editor and then I could try to derive information from it. It would break whenever it encounters an "unknown" process though.

We have a basic JS implementation that does this in the Earth Engine driver, so it's in principle not impossible get at least to a basic implementation of it.