cmd/tailscale: combine serve and funnel for debug wip funnel stream model (#9169)

> **Note**
> Behind the `TAILSCALE_USE_WIP_CODE` flag

In preparing for incoming CLI changes, this PR merges the code path for the `serve` and `funnel` subcommands.

See the parent issue for more context.

The following commands will run in foreground mode when using the environment flag.
```
tailscale serve localhost:3000
tailscae funnel localhost:3000
```

Replaces #9134
Updates #8489

Signed-off-by: Tyler Smalley <tyler@tailscale.com>
Signed-off-by: Marwan Sulaiman <marwan@tailscale.com>
Co-authored-by: Marwan Sulaiman <marwan@tailscale.com>

ipn/serve.go

@@ -93,6 +93,10 @@ type ServeStreamRequest struct {
 	// MountPoint is the path prefix for
 	// the given HostPort.
 	MountPoint string `json:",omitempty"`
+
+	// Funnel indicates whether the request
+	// is a serve request or a funnel one.
+	Funnel bool `json:",omitempty"`
 }
 
 // FunnelRequestLog is the JSON type written out to io.Writers