Add utility wrappers for instantiating builders/nodes

Add utility wrappers for instantiating builders/nodes

This removes the need for subclasses of
AbstractNodeContainerModificationStrategy to provide separate
methods, moving through a dispatch class instead. This lowers
the number of potential implementations down to one, with
the functionality being supported by 2 distinct classes.

As it turns out, this refactor actually has no additional overhead,
because the support instance reference can completely supplant
the Class reference we are holding.

JIRA: YANGTOOLS-941
Change-Id: I53a26981d41fb0cc6dd71dffd4e1bd71f9e9387d
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit c91afd55c080cf25473767536b4509ae3b5e2603)