Add default firewall rule to allow netbird traffic (#1056)

Add a default firewall rule to allow netbird traffic to be handled 
by the access control managers.

Userspace manager behavior:
- When running on Windows, a default rule is add on Windows firewall
- For Linux, we are using one of the Kernel managers to add a single rule
- This PR doesn't handle macOS

Kernel manager behavior:
- For NFtables, if there is a filter table, an INPUT rule is added
- Iptables follows the previous flow if running on kernel mode. If running 
on userspace mode, it adds a single rule for INPUT and OUTPUT chains

A new checkerFW package has been introduced to consolidate checks across
route and access control managers.
It supports a new environment variable to skip nftables and allow iptables tests

client/internal/routemanager/iptables_linux.go

@@ -49,29 +49,28 @@ type iptablesManager struct {
 	mux        sync.Mutex
 }
 
-func newIptablesManager(parentCtx context.Context) (*iptablesManager, error) {
+func newIptablesManager(parentCtx context.Context, ipv6Supported bool) (*iptablesManager, error) {
 	ipv4Client, err := iptables.NewWithProtocol(iptables.ProtocolIPv4)
 	if err != nil {
-		return nil, err
-	} else if !isIptablesClientAvailable(ipv4Client) {
-		return nil, fmt.Errorf("iptables is missing for ipv4")
-	}
-	ipv6Client, err := iptables.NewWithProtocol(iptables.ProtocolIPv6)
-	if err != nil {
-		log.Debugf("failed to initialize iptables for ipv6: %s", err)
-	} else if !isIptablesClientAvailable(ipv6Client) {
-		log.Infof("iptables is missing for ipv6")
-		ipv6Client = nil
+		return nil, fmt.Errorf("failed to initialize iptables for ipv4: %s", err)
 	}
 
 	ctx, cancel := context.WithCancel(parentCtx)
-	return &iptablesManager{
+	manager := &iptablesManager{
 		ctx:        ctx,
 		stop:       cancel,
 		ipv4Client: ipv4Client,
-		ipv6Client: ipv6Client,
 		rules:      make(map[string]map[string][]string),
-	}, nil
+	}
+
+	if ipv6Supported {
+		manager.ipv6Client, err = iptables.NewWithProtocol(iptables.ProtocolIPv6)
+		if err != nil {
+			log.Warnf("failed to initialize iptables for ipv6: %s. Routes for this protocol won't be applied.", err)
+		}
+	}
+
+	return manager, nil
 }
 
 // CleanRoutingRules cleans existing iptables resources that we created by the agent
@@ -486,8 +485,3 @@ func getIptablesRuleType(table string) string {
 	}
 	return ruleType
 }
-
-func isIptablesClientAvailable(client *iptables.IPTables) bool {
-	_, err := client.ListChains("filter")
-	return err == nil
-}