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/checkfw/check.go

@@ -0,0 +1,3 @@
+//go:build !linux
+
+package checkfw

client/internal/checkfw/check_linux.go

@@ -0,0 +1,56 @@
+//go:build !android
+
+package checkfw
+
+import (
+	"os"
+
+	"github.com/coreos/go-iptables/iptables"
+	"github.com/google/nftables"
+)
+
+const (
+	// UNKNOWN is the default value for the firewall type for unknown firewall type
+	UNKNOWN FWType = iota
+	// IPTABLES is the value for the iptables firewall type
+	IPTABLES
+	// IPTABLESWITHV6 is the value for the iptables firewall type with ipv6
+	IPTABLESWITHV6
+	// NFTABLES is the value for the nftables firewall type
+	NFTABLES
+)
+
+// SKIP_NFTABLES_ENV is the environment variable to skip nftables check
+const SKIP_NFTABLES_ENV = "NB_SKIP_NFTABLES_CHECK"
+
+// FWType is the type for the firewall type
+type FWType int
+
+// Check returns the firewall type based on common lib checks. It returns UNKNOWN if no firewall is found.
+func Check() FWType {
+	nf := nftables.Conn{}
+	if _, err := nf.ListChains(); err == nil && os.Getenv(SKIP_NFTABLES_ENV) != "true" {
+		return NFTABLES
+	}
+
+	ip, err := iptables.NewWithProtocol(iptables.ProtocolIPv4)
+	if err == nil {
+		if isIptablesClientAvailable(ip) {
+			ipSupport := IPTABLES
+			ipv6, ip6Err := iptables.NewWithProtocol(iptables.ProtocolIPv6)
+			if ip6Err == nil {
+				if isIptablesClientAvailable(ipv6) {
+					ipSupport = IPTABLESWITHV6
+				}
+			}
+			return ipSupport
+		}
+	}
+
+	return UNKNOWN
+}
+
+func isIptablesClientAvailable(client *iptables.IPTables) bool {
+	_, err := client.ListChains("filter")
+	return err == nil
+}