add AS-SET for route servers

[grizz]

Should probably wait until we pull prefixctl into the open source space and use that.

[telsin]

HE and others are now using this, so it's going to be important to be able to at least generate it and optimally auto-maintain it at your RIR of choice.

side note: at least Lumen have trouble with AS-SETs that reference AS-SETs at other IRRs. Not sure there's a good solution, but something to be aware of.