Allow exposing the inputs of a process function

[sphuber]

The exposing functionality of a the WorkChain class is very useful to allow writing workflows that wrap subprocesses such as other WorkChains and CalcJobs. However, this behavior is not yet supported for process functions and so we have to resort to manually copying the input specifications. Since process functions have an associated Process implementation that is generated dynamically, we could try to fetch its ProcessSpec and allow exposing its inputs.

[sphuber]

Note that this functionality is in fact already supported. However, its usefulness is limited by the fact that the exposed spec only inherits the port names (from the function arguments) but there is no additional information on valid type or help string message. #5900 implements a change where at least the valid type of the port can be inferred from the type hint of the function argument, if present. In similar fashion, the help message may be parsed from the function's docstring. A PR for this is in progress.