manifest: move optional modules to a submanifest and make them optional

Move optional modules to a submanifest and make them optional by
default.
This is just a POC, might require some more thought. The idea is to look
at the optional manifest as an area for modules that work with Zephyr,
but not needed directly by zephyr. This could be documented somewhere
for discovery purposes allowing users to enable such modules in their
downstream if desired.

See #54276

Signed-off-by: Anas Nashif <anas.nashif@intel.com>