Add non-null getters for leaf objects

Add non-null getters for leaf objects

DTO getters inherently return nullable objects. We have a nonnullFoo()
concept, which is very powerful with Lists/Maps, where we return an
empty collection. This turns out to be very useful for end users, who
can worry about other logic problems instead.

This patch introduces a similar construct for non-complex getters, so
that

  container foo {
    leaf bar {
      type string;
    }
  }

ends up generating a default method:

  interface Foo {
    default @NonNull String requireBar() {
        // ..
    }
  }

which nicely throws an exception distinct from NPE and retaining call
site.

JIRA: MDSAL-602
Change-Id: Ia66ee61f51fb9b6eb11aea3736e5246aadad6a3e
Signed-off-by: Kostiantyn Nosach <kostiantyn.nosach@pantheon.tech>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>