doc/netlink: Add sub-message support to netlink-raw

Add a 'sub-message' attribute type with a selector that supports
polymorphic attribute formats for raw netlink families like tc.

A sub-message attribute uses the value of another attribute as a
selector key to choose the right sub-message format. For example if the
following attribute has already been decoded:

  { "kind": "gre" }

and we encounter the following attribute spec:

  -
    name: data
    type: sub-message
    sub-message: linkinfo-data-msg
    selector: kind

Then we look for a sub-message definition called 'linkinfo-data-msg' and
use the value of the 'kind' attribute i.e. 'gre' as the key to choose
the correct format for the sub-message:

  sub-messages:
    name: linkinfo-data-msg
    formats:
      -
        value: bridge
        attribute-set: linkinfo-bridge-attrs
      -
        value: gre
        attribute-set: linkinfo-gre-attrs
      -
        value: geneve
        attribute-set: linkinfo-geneve-attrs

This would decode the attribute value as a sub-message with the
attribute-set called 'linkinfo-gre-attrs' as the attribute space.

A sub-message can have an optional 'fixed-header' followed by zero or
more attributes from an attribute-set. For example the following
'tc-options-msg' sub-message defines message formats that use a mixture
of fixed-header, attribute-set or both together:

  sub-messages:
    -
      name: tc-options-msg
      formats:
        -
          value: bfifo
          fixed-header: tc-fifo-qopt
        -
          value: cake
          attribute-set: tc-cake-attrs
        -
          value: netem
          fixed-header: tc-netem-qopt
          attribute-set: tc-netem-attrs

Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20231215093720.18774-3-donald.hunter@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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