Gateway v2 UI requirements

[Kodylow]

Requirement 1: setting for Relative fee should be configurable between 0 and 1000. It's done as parts per million. base fee always 1 sat. fedimint/fedimint#6052 . copy paste documentation from that pr. 3 types of fees in struct RoutingInfo.

Requirement 2: setting the gateway list per guardian. Each guardian should be able to add a gateway by URL and potentially be able to see and auto-add the gateways listed by the other guardians.