fix: 💊 re-export APIs used by Outline Client (#122)

Outline Client is referencing the following APIs (Android JNI interface as an example) exported by `gomobile`.

<img width="642" alt="image" src="https://user-images.githubusercontent.com/93548144/232151618-de7c91eb-2ee6-444e-bbe0-023262af2149.png">

But due to the recent refactoring, we introduced types that are not recognized by `gomobile`. This is the only function exported in `master` branch now:

<img width="646" alt="image" src="https://user-images.githubusercontent.com/93548144/232152396-6cf278ab-f45b-448a-adb9-c03b13772eb7.png">

In this PR, I re-exported these functions (together with the new `NewClientFromJSON`) by using the built-in types. Here is the exported Android JNI interface:

<img width="635" alt="image" src="https://user-images.githubusercontent.com/93548144/232151977-ad6bd330-4bc5-4383-b6ef-49dce4dc1853.png">

/cc @sbruens , although the required APIs are re-exported, some of the data types still need to be updated:

* [`OutlineTunnel`](https://github.com/Jigsaw-Code/outline-client/blob/master/src/cordova/android/OutlineAndroidLib/outline/src/main/java/org/outline/vpn/VpnTunnel.java#L49) -> `Tunnel`
* [`Tun2socksOutlineTunnel`](https://github.com/Jigsaw-Code/outline-client/blob/master/src/cordova/apple/OutlineAppleLib/Sources/PacketTunnelProviderSources/PacketTunnelProvider.m#L39) -> `Tun2socksTunnel`

Related PR: #118

outline/electron/main.go

@@ -103,7 +103,7 @@ func main() {
 		if err != nil {
 			log.Errorf("Failed to perform connectivity checks: %v", err)
 		}
-		os.Exit(connErrCode.Number())
+		os.Exit(connErrCode)
 	}
 
 	// Open TUN device

outline/shadowsocks/client.go

@@ -28,14 +28,13 @@ import (
 	"github.com/Jigsaw-Code/outline-go-tun2socks/outline"
 	"github.com/Jigsaw-Code/outline-go-tun2socks/outline/connectivity"
 	"github.com/Jigsaw-Code/outline-go-tun2socks/outline/internal/utf8"
-	"github.com/Jigsaw-Code/outline-go-tun2socks/outline/neterrors"
 	"github.com/Jigsaw-Code/outline-internal-sdk/transport"
 	"github.com/Jigsaw-Code/outline-internal-sdk/transport/shadowsocks"
 	"github.com/eycorsican/go-tun2socks/common/log"
 )
 
 // A client object that can be used to connect to a remote Shadowsocks proxy.
-type Client = outline.Client
+type Client outline.Client
 
 // NewClient creates a new Shadowsocks client from a non-nil configuration.
 //
@@ -96,17 +95,35 @@ func newShadowsocksClient(host string, port int, cipherName, password string, pr
 		return nil, fmt.Errorf("failed to create PacketListener: %w", err)
 	}
 
-	return &outline.Client{StreamDialer: streamDialer, PacketListener: packetListener}, nil
+	return &Client{StreamDialer: streamDialer, PacketListener: packetListener}, nil
 }
 
+// Error number constants exported through gomobile
+const (
+	NoError                     = 0
+	Unexpected                  = 1
+	NoVPNPermissions            = 2 // Unused
+	AuthenticationFailure       = 3
+	UDPConnectivity             = 4
+	Unreachable                 = 5
+	VpnStartFailure             = 6  // Unused
+	IllegalConfiguration        = 7  // Electron only
+	ShadowsocksStartFailure     = 8  // Unused
+	ConfigureSystemProxyFailure = 9  // Unused
+	NoAdminPermissions          = 10 // Unused
+	UnsupportedRoutingTable     = 11 // Unused
+	SystemMisconfigured         = 12 // Electron only
+)
+
 const reachabilityTimeout = 10 * time.Second
 
 // CheckConnectivity determines whether the Shadowsocks proxy can relay TCP and UDP traffic under
 // the current network. Parallelizes the execution of TCP and UDP checks, selects the appropriate
 // error code to return accounting for transient network failures.
 // Returns an error if an unexpected error ocurrs.
-func CheckConnectivity(client *Client) (neterrors.Error, error) {
-	return connectivity.CheckConnectivity(client)
+func CheckConnectivity(client *Client) (int, error) {
+	errCode, err := connectivity.CheckConnectivity((*outline.Client)(client))
+	return errCode.Number(), err
 }
 
 // CheckServerReachable determines whether the server at `host:port` is reachable over TCP.