tools/net/ynl: Refactor fixed header encoding into separate method

Refactor the fixed header encoding into a separate _encode_struct method
so that it can be reused for fixed headers in sub-messages and for
encoding structs.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20240129223458.52046-5-donald.hunter@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>