Add (DOM)DataTreeIdentifier serialization proxies

Add (DOM)DataTreeIdentifier serialization proxies

Further evolution of these two classes will require allowing for two
distinct specializations.

In order to be able to do that, we need to disconnect them from their
serial form.

This patch instroduces serialization proxies, which know how to write
out and read in these objects without referencing their class name.

JIRA: MDSAL-845
Change-Id: I2b50d3aa5f752d6181dab991a5a8282b079b16df
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit 9286037f2852d197c4384511fce97c551e2b8dc3)