ospf: support for opaque message

Signed-off-by: ISHIDA Wataru <ishida.wataru@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
ospf: fix bug in encoding

Signed-off-by: ISHIDA Wataru <ishida.wataru@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
ospf: add segment routing opaque types

Signed-off-by: ISHIDA Wataru <ishida.wataru@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
ospf: describe prefix in one attribute 'prefix' for ease of use

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