tools/ynl: Add mcast-group schema parsing to ynl

Add a SpecMcastGroup class to the nlspec lib.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/20230825122756.7603-6-donald.hunter@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>