tools: ynl-gen: submsg: render the structs

tools: ynl-gen: submsg: render the structs

The easiest (or perhaps only sane) way to support submessages in C
is to treat them as if they were nests. Build fake attributes to
that effect in the codegen. Render the submsg as a big nest of all
possible values.

With this in place the main missing part is to hook in the switch
which selects how to parse based on the key.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250515231650.1325372-6-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>