Create a UOM Conversion such that we don't have to create the conversion of UOM individually

Summary: We have a requirement such that when a primary UOM and a secondary UOM of an item are identified after doing a conversion for the first time, we should not be creating the same primary and secondary UOM again with a different conversion to avoid clogging the system with too many UOMs. For example:

• Primary UOM is carton.
• Secondary UOM is roll, which can have different values.

When a user is creating an item and selects the primary UOM as carton from a dropdown, the secondary UOM should provide a list of available secondary unit values like roll (21), roll (18)