Add a SDK_VERSION file pointing to the current SDK version.
This is meant to be used by upstream and downstream CIs to automatically
determined the current version of SDK based on the upstream checked out
version, as well as the documentation references, and intended to stay
in sync with the version used in the main CI.
Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>
Stephanos Ioannidis