[Feature Request]: Add Mapping (sub-transformation) / Mapping (sub-pipeline)

### What would you like to happen?

I'd like to request the following step/transform that is present in PDI
Mapping (sub-transformation)
Currently only those are present in Apache HOP

**We use "mapping (sub-transformation) in several of our projects since it supports more than one "Mapping input"**

Further i should note that if the import tool is used to import transformations to Apache HOP all "Mapping (sub-transformations)" will be replaced by a "Simple-Mapping (sup-pipeline)".
If more then one "Mapping input" is used on those you get a HopExeption:

To be able to import all our projects we would need the step "Mapping (sub-transformation)"

### Issue Priority

Priority: 2

### Issue Component

Component: Import, Component: Transforms

Start by tracing the import tool's handling of Mapping (sub-transformation) and Simple-Mapping (sub-pipeline), then review the existing transform components. The work is done when imports preserve Mapping (sub-transformation), support more than one Mapping input, and no longer produce the reported HopException.

Written by the indexing model from the issue text.