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/nftables_linux.go

@@ -86,10 +86,10 @@ type nftablesManager struct {
 	mux                 sync.Mutex
 }
 
-func newNFTablesManager(parentCtx context.Context) (*nftablesManager, error) {
+func newNFTablesManager(parentCtx context.Context) *nftablesManager {
 	ctx, cancel := context.WithCancel(parentCtx)
 
-	mgr := &nftablesManager{
+	return &nftablesManager{
 		ctx:                 ctx,
 		stop:                cancel,
 		conn:                &nftables.Conn{},
@@ -97,18 +97,6 @@ func newNFTablesManager(parentCtx context.Context) (*nftablesManager, error) {
 		rules:               make(map[string]*nftables.Rule),
 		defaultForwardRules: make([]*nftables.Rule, 2),
 	}
-
-	err := mgr.isSupported()
-	if err != nil {
-		return nil, err
-	}
-
-	err = mgr.readFilterTable()
-	if err != nil {
-		return nil, err
-	}
-
-	return mgr, nil
 }
 
 // CleanRoutingRules cleans existing nftables rules from the system
@@ -147,6 +135,10 @@ func (n *nftablesManager) RestoreOrCreateContainers() error {
 	}
 
 	for _, table := range tables {
+		if table.Name == "filter" {
+			n.filterTable = table
+			continue
+		}
 		if table.Name == nftablesTable {
 			if table.Family == nftables.TableFamilyIPv4 {
 				n.tableIPv4 = table
@@ -259,21 +251,6 @@ func (n *nftablesManager) refreshRulesMap() error {
 	return nil
 }
 
-func (n *nftablesManager) readFilterTable() error {
-	tables, err := n.conn.ListTables()
-	if err != nil {
-		return err
-	}
-
-	for _, t := range tables {
-		if t.Name == "filter" {
-			n.filterTable = t
-			return nil
-		}
-	}
-	return nil
-}
-
 func (n *nftablesManager) eraseDefaultForwardRule() error {
 	if n.defaultForwardRules[0] == nil {
 		return nil
@@ -544,14 +521,6 @@ func (n *nftablesManager) removeRoutingRule(format string, pair routerPair) erro
 	return nil
 }
 
-func (n *nftablesManager) isSupported() error {
-	_, err := n.conn.ListChains()
-	if err != nil {
-		return fmt.Errorf("nftables is not supported: %s", err)
-	}
-	return nil
-}
-
 // getPayloadDirectives get expression directives based on ip version and direction
 func getPayloadDirectives(direction string, isIPv4 bool, isIPv6 bool) (uint32, uint32, []byte) {
 	switch {