chore(gui-client/linux): fix DNS (#4802)

Make the GUI use systemd-resolved to retrieve the system's resolvers.
This allows the IPC service to set up sentinels for those resolvers and
control the system's DNS.

Closes #3812

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

@@ -7,12 +7,8 @@ mod imp {
     use anyhow::Result;
     use std::net::IpAddr;
 
-    // TODO: The code here will depend on the chosen DNS control method.
-    // So that will need to be threaded in here somehow.
-    #[allow(clippy::unnecessary_wraps)]
     pub fn get() -> Result<Vec<IpAddr>> {
-        tracing::error!("Resolvers module not yet implemented for Linux, returning empty Vec");
-        Ok(Vec::default())
+        firezone_headless_client::imp::get_system_default_resolvers_systemd_resolved()
     }
 }
 

rust/headless-client/src/imp_linux.rs

@@ -139,7 +139,7 @@ fn get_system_default_resolvers_network_manager() -> Result<Vec<IpAddr>> {
 }
 
 /// Returns the DNS servers listed in `resolvectl dns`
-fn get_system_default_resolvers_systemd_resolved() -> Result<Vec<IpAddr>> {
+pub fn get_system_default_resolvers_systemd_resolved() -> Result<Vec<IpAddr>> {
     // Unfortunately systemd-resolved does not have a machine-readable
     // text output for this command: <https://github.com/systemd/systemd/issues/29755>
     //