tools: ynl-gen: support weird sub-message formats

TC uses all possible sub-message formats:
 - nested attrs
 - fixed headers + nested attrs
 - fixed headers
 - empty

Nested attrs are already supported for rt-link. Add support
for remaining 3. The empty and fixed headers ones are fairly
trivial, we can fake a Binary or Flags type instead of a Nest.

For fixed headers + nest we need to teach nest parsing and
nest put to handle fixed headers.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250520161916.413298-10-kuba@kernel.org


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