227: Assign supplying depot to requisitions for "Convert to Order"

Short description: 

• Provide API endpoint(s) to retrieve all valid supplying depots for a given approved requisition
  • valid supplying depots - look into the logic in v2 to determine which supplying depots should be considered valid (I think some combination of supply line/supervisory node/program) Please update the acceptance criteria once it is identified
• can select the supplying depot (depots are previously defined and sit in reference data) and assign it to a specific approved requisition (UI)
• once supply depot is assigned to the requisition, the user can 'convert to order' (process of converting an order is addressed in another story OLMIS-226 Done ) (may be mainly in the UI)
  • convert to order process should retrieve the supply depot selected
• receive an error message if they attempt to convert an order without selecting a supply depot
• can select and assign supplying depots to multiple requisitions at once (then all will be converted)
• the system pre-populates the supply depot when there is only one depot associated with the requisition in the system (not sure if this is just within the UI or if this can be implemented in the API calls)
• appropriate user permissions are enforced