Add ExtensibleObject interface
This adds the basic ExtensibleObject and ObjectExtension interfaces,
which can be reused to define extensible objects.
Unlike a full-blown implementation, these interfaces provide only
access interface, not a mechanism to perform actual state attachment.
The attachment mechanism will be defined in future, when the need
for it actually arises.
JIRA: YANGTOOLS-497
Change-Id: Ie390b6174b8909c87595dddc0d467858f36ef8bf
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>