Implement the 'sub-message' attribute type in ynl.

Encode support is not yet implemented. Support for sub-message selectors
at a different nest level from the key attribute is not yet supported.
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20231215093720.18774-5-donald.hunter@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>