Add yang-binding interfaces for annotations

Binding users are interested in having a way to access RFC7952
metadata. While the actual codegen/runtime implementation requires
some work, we know what Binding Specification extension we will
need to support the generated classes.

This patch introduces Annotation and AnnotationAware, so a future
codegen version can bind to it.

JIRA: MDSAL-357
Change-Id: I2fed30ea34b21c36b626388bb37890cc73875176
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>