ofp-msgs: Add support for ONF extension messages.

ofp-msgs: Add support for ONF extension messages.

ONF introduced a number of "standard extensions" that use its own
vendor (experimenter) ID.  This commit adds support for such extensions to
ofp-msgs.

These extensions were already half-supported, so there's barely any change
to build-aux/extract-ofp-msgs.

This isn't fully tested, since nothing adds support for such a message yet.

Requested-by: Jarno Rajahalme <jarno@ovn.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Jarno Rajahalme <jarno@ovn.org>