Allow user to get proposal codes/instrument session

It would be useful if the user could request what proposal codes are valid.

This could also then be used to check whether the submit_workflows is submitting against a valid proposal code

Start by reading the submit_workflows entry point and the surrounding issue context to determine how proposal codes and instrument sessions are currently represented. Done means users can request valid proposal codes and submissions can check whether the selected proposal code is valid.

Scritto dal modello di indicizzazione a partire dal testo della issue.