Replace GroupingResolver with AugmentationGroupingResolver

Replace GroupingResolver with AugmentationGroupingResolver

AugmentationGroupingResolver provides the same functionality with
an immutable implementation, geared towards low memory overhead
both in terms of static allocation and memory usage during traversal.

GroupingResolver.getExtension() functionality is exposed as
findExtension(), returning java.util.Optional -- thus allowing
for a clean option to retrofit a @Nullable getter if the need
arises.

JIRA: OPNFLWPLUG-1050
Change-Id: I7346ff41c6b1425a9f95dccb11547836f0a15c53
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>