Allow Setting Explicit Relay(s) Based on Device Group

[misterpatch]

We would like to see the option to set an explicit relay (or relays) based on the Device Group configured in RustDesk Server Pro.

We have two datacenters with on-prem devices, one in New York City and the other in Los Angeles. We need to ensure that our users are connecting using the lowest latency path possible. We recognize that there is an option to use Geo Location for automatic relay selection, however this doesn't work well when a user is between two geographically distant relays.

For example, if a user in Chicago uses Geo Location to select a relay, they will always connect to New York even if the RustDesk device they want to access is in Los Angeles, thus increasing their latency.

The benefit of using a Device Group instead of configuring a Custom Client is that it is dynamic and reduces the complexity so that a user only requires one Custom Client application.