Make it possible for struct definitions to reference other struct
definitions ofr binary members. For example, the tbf qdisc uses this
struct definition for its parms attribute:

  -
    name: tc-tbf-qopt
    type: struct
    members:
      -
        name: rate
        type: binary
        struct: tc-ratespec
      -
        name: peakrate
        type: binary
        struct: tc-ratespec
      -
        name: limit
        type: u32
      -
        name: buffer
        type: u32
      -
        name: mtu
        type: u32

This adds the necessary schema changes and adds nested struct encoding
and decoding to ynl.
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20240129223458.52046-11-donald.hunter@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>