bluetooth: mesh: health_cli: Add callback to pull msg for periodic pub

The Health Client model supports periodic publication and thus requires
the update callback to be initialized.

This commit implements a callback inside the health client model and
adds a wrapper to it so that the user can fill up the msg buffer.
This way, the Config Server will always accept
Config Model Publication Set message with periodic publication
parameters. If the users callback returns an error or is not
implemented, the periodic publication will be aborted.

Signed-off-by: Stine Åkredalen <stine.akredalen@nordicsemi.no>