tools: ynl: add Python API for easier access to policies

tools: ynl: add Python API for easier access to policies

The format of Netlink policy dump is a bit curious with messages
in the same dump carrying both attrs and mapping info. Plus each
message carries a single piece of the puzzle the caller must then
reassemble.

I need to do this reassembly for a test, but I think it's generally
useful. So let's add proper support to YnlFamily to return more
user-friendly representation. See the various docs in the patch
for more details.

Link: https://patch.msgid.link/20260310005337.3594225-5-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>