Schema change / purl-type-definition: Add enum for qualifiers keys

[mjherzog]

There is a current proposal to change schemas/purl-type-definition.schema-1.1.json and the deb and rpm PURL type definitions to add a list of enumerated values for the namespace PURL component.

• Issue Schema change / purl-type-definition: Add enum for namespace values #690
• PR Add enumeration to the namespace field; adding enums for deb/rpm #678

This issue is to propose a similar change for the qualifiers PURL component. The concept of enumerated values for qualifiers is essentially the same as for namespace and would also likely be relevant for the deb and rpm PURL types.