[Feature Request]: Make input and output optional in a mapping

### What would you like to happen?

Right now, the Simple Mapping transform looks for a "Mapping Input" transform (in the sub-pipeline) to see where it can send input to. Likewise, the same happens with the "Mapping Output" transform.
However, in some cases you just have output (you want to read from the same source multiple times in multiple pipelines) and in other cases you have multiple inputs that all write to the same target (without the need for output).

For these scenarios you'd need options in the "Input" and "Output" tabs of the "Simple Mapping" transform that say

[x] There is no input for this mapping.
[x] There is no output for this mapping.

### Issue Priority

Priority: 3

### Issue Component

Component: Transforms

Start with the Simple Mapping transform and inspect how its Input and Output tabs locate Mapping Input and Mapping Output transforms in the sub-pipeline. Determine how the new no-input and no-output options should affect those existing checks. Done means mappings can run with only output, only input, or both, while preserving current behavior when neither option is selected.

Written by the indexing model from the issue text.