Chaining Parameterized objects

The `@param.output` decorator allows a user to declare that their Parameterized object generates an output of the indicated type, which can be used by external programs like [Panel](https://panel.holoviz.org/user_guide/Pipelines.html) to connect outputs from one stage to inputs of another. It seems to me that Param should also use that information internally, both to validate that the output matches the declared type (proposed in https://github.com/holoviz/param/issues/515) and to somehow allow chaining outputs to inputs. E.g. we could have a function or method that we could call to connect one Parameterized to another, analogous to `add_stage` in Panel, which would set up one parameter on one object as being dependent on the output from another object. Precisely how to set up that linkage would take a bit of work, but it seems like the natural way to flesh out the output support to be useful at the Param level.