tools/net/ynl: Move formatted_string method out of NlAttr

The formatted_string() class method was in NlAttr so that it could be
accessed by NlAttr.as_struct(). Now that as_struct() has been removed,
move formatted_string() to YnlFamily as an internal helper method.

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/20240129223458.52046-10-donald.hunter@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>