netlink: specs: Add a spec for FIB rule management

Add a YNL spec for FIB rules:

./tools/net/ynl/cli.py \
    --spec Documentation/netlink/specs/rt_rule.yaml \
    --dump getrule --json '{"family": 2}'

[{'action': 'to-tbl',
  'dst-len': 0,
  'family': 2,
  'flags': 0,
  'protocol': 2,
  'src-len': 0,
  'suppress-prefixlen': '0xffffffff',
  'table': 255,
  'tos': 0},
  ... ]

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20241106090718.64713-3-donald.hunter@gmail.com


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