tools: ynl-gen: generate nested array policies

tools: ynl-gen: generate nested array policies

This patch adds support for NLA_POLICY_NESTED_ARRAY() policies.

Example spec (from future wireguard.yaml):
-
  name: wgpeer
  attributes:
    -
      name: allowedips
      type: indexed-array
      sub-type: nest
      nested-attributes: wgallowedip

yields NLA_POLICY_NESTED_ARRAY(wireguard_wgallowedip_nl_policy).

This doesn't change any currently generated code, as it isn't
used in any specs currently used for generating code.

Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20250915144301.725949-3-ast@fiberby.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>