Introduce support for importing a VSP which contains policy types which are specific to the package being imported, i.e. the types are previously unknown to the system

the new types are to be created in the DB and connected to a new model instance in the same way as is done for data types (https://gerrit.onap.org/r/c/sdc/+/123368)

Scope limited to policy types defined in the main template (i.e. no need for any handling of globalSubstitutes)