packet: define BGPPathAttr type

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
packet: support pretty PathAttribute JSON printing

some are missing though.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
table: update path struct JSON format

Print all path attributes in a path.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>