cmd/tailscale/cli, ipn/ipnlocal: [funnel] add stream mode

Adds ability to start Funnel in the foreground and stream incoming
connections. When foreground process is stopped, Funnel is turned
back off for the port.

Exampe usage:
```
TAILSCALE_FUNNEL_V2=on tailscale funnel 8080
```

Updates #8489

Signed-off-by: Marwan Sulaiman <marwan@tailscale.com>

ipn/serve.go

@@ -12,6 +12,7 @@ import (
 	"slices"
 	"strconv"
 	"strings"
+	"time"
 
 	"tailscale.com/tailcfg"
 )
@@ -42,6 +43,21 @@ type ServeConfig struct {
 // There is no implicit port 443. It must contain a colon.
 type HostPort string
 
+// Port extracts just the port number from hp.
+// An error is reported in the case that the hp does not
+// have a valid numeric port ending.
+func (hp HostPort) Port() (uint16, error) {
+	_, port, err := net.SplitHostPort(string(hp))
+	if err != nil {
+		return 0, err
+	}
+	port16, err := strconv.ParseUint(port, 10, 16)
+	if err != nil {
+		return 0, err
+	}
+	return uint16(port16), nil
+}
+
 // A FunnelConn wraps a net.Conn that is coming over a
 // Funnel connection. It can be used to determine further
 // information about the connection, like the source address
@@ -62,6 +78,41 @@ type FunnelConn struct {
 	Src netip.AddrPort
 }
 
+// ServeStreamRequest defines the json request body
+// for the serve stream endpoint
+type ServeStreamRequest struct {
+	// HostPort is the DNS and port of the tailscale
+	// URL.
+	HostPort HostPort `json:",omitempty"`
+
+	// Source is the user's serve destination
+	// such as their localhost server.
+	Source string `json:",omitempty"`
+
+	// MountPoint is the path prefix for
+	// the given HostPort.
+	MountPoint string `json:",omitempty"`
+}
+
+// FunnelRequestLog is the JSON type written out to io.Writers
+// watching funnel connections via ipnlocal.StreamServe.
+//
+// This structure is in development and subject to change.
+type FunnelRequestLog struct {
+	Time time.Time `json:",omitempty"` // time of request forwarding
+
+	// SrcAddr is the address that initiated the Funnel request.
+	SrcAddr netip.AddrPort `json:",omitempty"`
+
+	// The following fields are only populated if the connection
+	// initiated from another node on the client's tailnet.
+
+	NodeName        string   `json:",omitempty"` // src node MagicDNS name
+	NodeTags        []string `json:",omitempty"` // src node tags
+	UserLoginName   string   `json:",omitempty"` // src node's owner login (if not tagged)
+	UserDisplayName string   `json:",omitempty"` // src node's owner name (if not tagged)
+}
+
 // WebServerConfig describes a web server's configuration.
 type WebServerConfig struct {
 	Handlers map[string]*HTTPHandler // mountPoint => handler