TUN-8914: Add a new configuration to locally override the max-active-flows

## Summary

This commit introduces a new command line flag, `--max-active-flows`, which allows overriding the remote configuration for the maximum number of active flows.

The flag can be used with the `run` command, like `cloudflared tunnel --no-autoupdate run --token <TUNNEL_TOKEN> --max-active-flows 50000`, or set via an environment variable `TUNNEL_MAX_ACTIVE_FLOWS`.

Note that locally-set values always take precedence over remote settings, even if the tunnel is remotely managed.

Closes TUN-8914

cmd/cloudflared/tunnel/cmd.go

@@ -126,7 +126,7 @@ var (
 	routeFailMsg = fmt.Sprintf("failed to provision routing, please create it manually via Cloudflare dashboard or UI; "+
 		"most likely you already have a conflicting record there. You can also rerun this command with --%s to overwrite "+
 		"any existing DNS records for this hostname.", overwriteDNSFlag)
-	errDeprecatedClassicTunnel = fmt.Errorf("Classic tunnels have been deprecated, please use Named Tunnels. (https://developers.cloudflare.com/cloudflare-one/connections/connect-apps/install-and-setup/tunnel-guide/)")
+	errDeprecatedClassicTunnel = errors.New("Classic tunnels have been deprecated, please use Named Tunnels. (https://developers.cloudflare.com/cloudflare-one/connections/connect-apps/install-and-setup/tunnel-guide/)")
 	// TODO: TUN-8756 the list below denotes the flags that do not possess any kind of sensitive information
 	// however this approach is not maintainble in the long-term.
 	nonSecretFlagsList = []string{
@@ -214,6 +214,7 @@ var (
 		"protocol",
 		"overwrite-dns",
 		"help",
+		"max-active-flows",
 	}
 )