tools/net/ynl: Add support for nested structs

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.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>