signaldctl: init at 0.6.1

signaldctl is a tool used to administer the signald service (already
available in nixpkgs). after enabling the signald service as below:

```nix
{
  services.signald.enable = true;
}
```

then a user can use signaldctl to introspect signald's state.
the communication happens over `/run/signald/signald.sock`, so either
configure signald to expose that with desired permissions, or run
signaldctl as the signald user:
```
$ sudo -u signald -g signald signaldctl account list
┌──────────────┬──────────────────────────────────────┬───────────┐
│ PHONE NUMBER │ UUID                                 │ DEVICE ID │
├──────────────┼──────────────────────────────────────┼───────────┤
│ +1XXXXXXXXXX │ XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX │         1 │
└──────────────┴──────────────────────────────────────┴───────────┘
```

pkgs/applications/networking/instant-messengers/signaldctl/default.nix

@@ -0,0 +1,41 @@
+{ lib
+, buildGoModule
+, fetchFromGitLab
+, gitUpdater
+}:
+
+buildGoModule rec {
+  pname = "signaldctl";
+  version = "0.6.1";
+  src = fetchFromGitLab {
+    owner = "signald";
+    repo = "signald-go";
+    rev = "v${version}";
+    hash = "sha256-lMJyr4BPZ8V2f//CUkr7CVQ6o8nRyeLBHMDEyLcHSgQ=";
+  };
+
+  vendorHash = "sha256-LGIWAVhDJCg6Ox7U4ZK15K8trjsvSZm4/0jNpIDmG7I=";
+
+  installPhase = ''
+    runHook preInstall
+
+    mkdir -p $out/bin
+    # install only the binary and not any intermediate artifacts like
+    # `generators` which is only used during build
+    cp "$GOPATH/bin/signaldctl" $out/bin
+
+    runHook postInstall
+  '';
+
+  passthru.updateScript = gitUpdater {
+    rev-prefix = "v";
+  };
+
+  meta = with lib; {
+    description = "A golang library for communicating with signald";
+    homepage = "https://signald.org/signaldctl/";
+    license = licenses.gpl3;
+    maintainers = with maintainers; [ colinsane ];
+    platforms = platforms.unix;
+  };
+}

pkgs/top-level/all-packages.nix

@@ -11708,6 +11708,8 @@ with pkgs;
 
   signald = callPackage ../applications/networking/instant-messengers/signald { };
 
+  signaldctl = callPackage ../applications/networking/instant-messengers/signaldctl { };
+
   signal-cli = callPackage ../applications/networking/instant-messengers/signal-cli { };
 
   inherit (callPackage ../applications/networking/instant-messengers/signal-desktop {}) signal-desktop signal-desktop-beta;