Add possibility for optional inputs and input validation

[KZzizzle]

USER STORY:

I would like to be able to mark an input as optional or mandatory with input validation before running the service. Right now, if an input expects (for example) a file and nothing is linked, there is no error message but the pipeline doesn't run.

DEFINITION OF DONE:

A clear and concise description of what the feature requires.

0. Input validation based on the data type expected
1. option to specify if an input is optional
2. helpful error message if something in the sidecar does not work