TUN-4017: Add support for using cloudflared as a full socks proxy.

To use cloudflared as a socks proxy, add an ingress on the server
side with your desired rules. Rules are matched in the order they
are added.  If there are no rules, it is an implicit allow.  If
there are rules, but no rule matches match, the connection is denied.

ingress:
  - hostname: socks.example.com
    service: socks-proxy
    originRequest:
      ipRules:
        - prefix: 1.1.1.1/24
          ports: [80, 443]
          allow: true
        - prefix: 0.0.0.0/0
          allow: false

On the client, run using tcp mode:
cloudflared access tcp --hostname socks.example.com --url 127.0.0.1:8080

Set your socks proxy as 127.0.0.1:8080 and you will now be proxying
all connections to the remote machine.

ingress/origin_service.go

@@ -12,6 +12,7 @@ import (
 	"time"
 
 	"github.com/cloudflare/cloudflared/hello"
+	"github.com/cloudflare/cloudflared/ipaccess"
 	"github.com/cloudflare/cloudflared/socks"
 	"github.com/cloudflare/cloudflared/tlsconfig"
 	"github.com/cloudflare/cloudflared/websocket"
@@ -100,6 +101,10 @@ type tcpOverWSService struct {
 	streamHandler streamHandlerFunc
 }
 
+type socksProxyOverWSService struct {
+	conn *socksProxyOverWSConnection
+}
+
 func newTCPOverWSService(url *url.URL) *tcpOverWSService {
 	switch url.Scheme {
 	case "ssh":
@@ -122,6 +127,16 @@ func newBastionService() *tcpOverWSService {
 	}
 }
 
+func newSocksProxyOverWSService(accessPolicy *ipaccess.Policy) *socksProxyOverWSService {
+	proxy := socksProxyOverWSService{
+		conn: &socksProxyOverWSConnection{
+			accessPolicy: accessPolicy,
+		},
+	}
+
+	return &proxy
+}
+
 func addPortIfMissing(uri *url.URL, port int) {
 	if uri.Port() == "" {
 		uri.Host = fmt.Sprintf("%s:%d", uri.Hostname(), port)
@@ -144,6 +159,14 @@ func (o *tcpOverWSService) start(wg *sync.WaitGroup, log *zerolog.Logger, shutdo
 	return nil
 }
 
+func (o *socksProxyOverWSService) start(wg *sync.WaitGroup, log *zerolog.Logger, shutdownC <-chan struct{}, errC chan error, cfg OriginRequestConfig) error {
+	return nil
+}
+
+func (o *socksProxyOverWSService) String() string {
+	return ServiceSocksProxy
+}
+
 // HelloWorld is an OriginService for the built-in Hello World server.
 // Users only use this for testing and experimenting with cloudflared.
 type helloWorld struct {