Add diffing to CI workflow

[nightlark]

Currently the workflow only converts a single HELICS header file to json. It should also run the diff command.

The desired functionality for the CI workflow is:

• Use workflow_dispatch as a trigger that takes old and new versions as optional inputs
• Automatically trigger workflow_dispatch with old and new versions from the helics_packaging repository when a new HELICS version is released
• If old and/or new versions were not provided, automatically determine the latest two versions released
• Download HELICS C header file for each version
• Convert HELICS C header file for both versions to JSON
• Do a diff of the HELICS C header file for both versions and save the result

This functionality could be interesting to incorporate as a GitHub composite action, enabling it to be used in other repositories easily... then maybe it could be used directly in helics_packaging workflow instead, and automatically open issues in repositories for the various language interfaces detailing what changes need to be made?