tools/net/ynl: Support sub-messages in nested attribute spaces

Sub-message selectors could only be resolved using values from the
current nest level. Enable value lookup in outer scopes by using
collections.ChainMap to implement an ordered lookup from nested to
outer scopes.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20240129223458.52046-3-donald.hunter@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>