Add new MRT attributes

[phillip-stephens]

Changes

• Adds support for all MRT path attributes GoBGP supports in its latest version
• Print an INFO message for any path attributes GoBGP doesn't support, both RIPE RIS and RouteViews' latest MRT file contains one (see details below)
• Add a test using a minimal MRT file

Reviewer Note

I changed the behavior so that if an error is encountered when parsing a MRT body, we'll skip that MRT msg rather than aborting.

A couple RIB MRT's I pulled from RouteViews (and also from RIPE RIS) had an attribute that GoBGP didn't recognize, even with the latest version. It seems even subtype = 249 is used by some router vendors to implement custom logic since it's unassigned by IANA.

I think skipping these unparseable ones (and logging at info level) and continuing is the best we can do.

echo "1.1.1.1" | ./zannotate --routing --routing-mrt-file=/tmp/bview.20260301                                                                                     11:11:06
INFO[0029] failed to parse mrt body: unknown evpn subtype: 249