Allow to limit allowed subtypes instead of requiring explicit definition

[beikov]

During some actual usage of updatable entity views with subtypes, we came to the conclusion that it's best to allow subtypes for attributes by default rather than force users to declare allowed subtypes.
We still consider the statically declared type to determine if creatable or updatable subtypes are allowed.

The general idea is, if the static type is read-only, all read-only subtypes of the declared type are allowed.
If the type is creatable or the cascades contains PERSIST, we also allow all creatable subtypes.
If the type is updatable or the cascades contains UPDATE, we also allow all updatable subtypes.
If the subtypes are declared, we only allow the given set of subtypes.