topology: allow peering links between core-ASes

[matzf]

Currently, the topology loader forbids core ASes from having peering links (i.e. links with "link_to": "peer"). The validation check is here:

scion/private/topology/topology.go

Lines 544 to 550 in 4a8bc98

	if isCore {
	switch i.LinkType {
	case Core, Child:
	default:
	return serrors.New("Illegal link type for core AS",
	"type", i.LinkType, "br", brName)
	}

.
As far as I'm aware, there is no technical reason why core ASes cannot have peering links.
Core ASes should be able to have peering links to any other AS, i.e. to both core or non-core ASes in both the same or different ISDs.

[nicorusti]

In the current definitions of peering links in IETF drafts we say:

Peering Link: A link between two SCION border routers of different
ASes, where at least one of the two ASes is not core.

@jiceatscion We should clarify this and how it reflects into the spec

[jiceatscion]

Both Anapaya and Adrian confirm that there's no reason to prohibit peering between core ASes. So my update to the draft should stand. As for fixing the code, that's a pretty minor issue comparatively. Just change the snippet that Matthias copied here and the associated tests if any.