Merge branch 'tools-ynl-gen-support-sub-messages-and-rt-link'

Jakub Kicinski says:

====================
tools: ynl-gen: support sub-messages and rt-link

Sub-messages are how we express "polymorphism" in YNL. Donald added
the support to specs and Python a while back, support them in C, too.
Sub-message is a nest, but the interpretation of the attribute types
within that nest depends on a value of another attribute. For example
in rt-link the "kind" attribute contains the link type (veth, bonding,
etc.) and based on that the right enum has to be applied to interpret
link-specific attributes.

The last message is probably the most interesting to look at, as it
adds a fairly advanced sample.

This patch only contains enough support for rtnetlink, we will need
a little more complexity to support TC, where sub-messages may contain
fixed headers, and where the selector may be in a different nest than
the submessage.
====================

Link: https://patch.msgid.link/20250515231650.1325372-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>