Bluetooth: Mesh: Refactor public headers into dedicated subdirectory

As the number of mesh APIs grows it becomes a bit cumbersome to have
everything in a single header file. Split the mesh.h header file into
multiple files in a new mesh subdirectory, and include the new headers
from the old one to retain backwards compatibility and simplicity for
apps (they only need to include <bluetooth/mesh.h>).

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>