cli: add service integration for systemd (#166328)

systemd, like most 'modern' linux components, has a nice dbus API. This uses
that API to register the tunnel as a service of the calling user.

The dbus dependency is temporarily duplicated, until secret-service 3 is
released, where they update to the latest version (should be a week or two).

For https://github.com/microsoft/vscode-cli/issues/367. Next up, macOS,
then it's done :)

cli/Cargo.toml

@@ -53,6 +53,7 @@ winreg = "0.10"
 
 [target.'cfg(target_os = "linux")'.dependencies]
 tar = { version = "0.4" }
+zbus = { version = "3.4", default-features = false, features = ["tokio"] }
 
 [patch.crates-io]
 russh = { git = "https://github.com/microsoft/vscode-russh", branch = "main" }