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/subcommands.go

@@ -230,6 +230,11 @@ var (
 		Usage: "Network diagnostics won't be performed",
 		Value: false,
 	}
+	maxActiveFlowsFlag = &cli.Uint64Flag{
+		Name:    "max-active-flows",
+		Usage:   "Overrides the remote configuration for max active private network flows (TCP/UDP) that this cloudflared instance supports",
+		EnvVars: []string{"TUNNEL_MAX_ACTIVE_FLOWS"},
+	}
 )
 
 func buildCreateCommand() *cli.Command {
@@ -705,6 +710,7 @@ func buildRunCommand() *cli.Command {
 		tunnelTokenFlag,
 		icmpv4SrcFlag,
 		icmpv6SrcFlag,
+		maxActiveFlowsFlag,
 	}
 	flags = append(flags, configureProxyFlags(false)...)
 	return &cli.Command{