Route Server / other metadata support

[compuzip]

Do we want to eventually support route servers in any sort of "different" way? Realistically, peering with a route server isn't much different than peering with any other ASN, but having specific signaling could indicate something special to config generation logic that happens on the other end. @grizz points out next-hop-self - although now I'm wondering if any other metadata would be interesting. From a CDN perspective, "who you should expect to serve through here" could be useful, but that may even be useful in a non-RS case.

[tking]

Many IXPs pre-configure Route Server sessions. So, it could be straightforward to integrate IXP Route Servers into the API.