fix(connlib): retain order of system/upstream DNS servers (#10773)

Right now, connlib hands out a `BiMap` of sentinel IPs <> upstream
servers whenever it emits a `TunInterfaceUpdated` event. This `BiMap`
internally uses two `HashMap`s. The iteration order of `HashMap`s is
non-deterministic and therefore, we lose the order in which the upstream
/ system resolvers have been passed to us originally.

To prevent that, we now emit a dedicated `DnsMapping` type that does not
expose its internal data structure but only getters for retrieving the
sentinel and upstream servers. Internally, it uses a `Vec` to store this
mapping and thus retains the original order. This is asserted as part of
our proptests by comparing the resulting `Vec`s.

This fix is preceded by a few refactorings that encapsulate the code for
creating and updating this DNS mapping.

Resolves: #8439

rust/gui-client/src-tauri/src/service.rs

@@ -495,7 +495,7 @@ impl<'a> Handler<'a> {
 
                 self.tun_device.set_ips(config.ip.v4, config.ip.v6).await?;
                 self.dns_controller
-                    .set_dns(config.dns_sentinel_ips(), config.search_domain)
+                    .set_dns(config.dns_by_sentinel.sentinel_ips(), config.search_domain)
                     .await?;
                 self.tun_device
                     .set_routes(config.ipv4_routes, config.ipv6_routes)