tools: ynl-gen: submsg: support parsing and rendering sub-messages

Adjust parsing and rendering appropriately to make sub-messages work.
Rendering is pretty trivial, as the submsg -> netlink conversion looks
like rendering a nest in which only one attr was set. Only trick
is that we use the enum value of the sub-message rather than the nest
as the type, and effectively skip one layer of nesting. A real double
nested struct would look like this:

  [SELECTOR]
  [SUBMSG]
    [NEST]
      [MSG1-ATTR]

A submsg "is" the nest so by skipping I mean:

  [SELECTOR]
  [SUBMSG]
    [MSG1-ATTR]

There is no extra validation in YNL if caller has set the selector
matching the submsg type (e.g. link type = "macvlan" but the nest
attrs are set to carry "veth"). Let the kernel handle that.

Parsing side is a little more specialized as we need to render and
insert a new kind of function which switches between what to parse
based on the selector. But code isn't too complicated.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250515231650.1325372-7-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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