Add two utilities to obtain inputs from the action: getOptionalInput and getRequiredInput.

These achieve two things. First, we have a more consistent use of undefined when optional inputs are not provided. Second, we provide a better semantic characterization of which inputs are optional or required (ensuring that we do throw if required inputs are not given).

The changes mostly affect the action and not the runner (that already has a similar semantics).

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.