Support for System Proposal User Fields

Description: Add the ability to Add/Edit/Delete the Proposal User field from the system. Make sure to have the ability to update proposal user field values.

Use Case/Example: The ability to update the system fields and have the new values be available to the proposal users. I have a list of proposal identifiers in the external systems, and I want to be able to update the list of values for a proposal user field to keep the list in ProPricer in sync with the external system.