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/firewall_nonlinux.go

@@ -6,9 +6,10 @@ package routemanager
 import (
 	"context"
 	"fmt"
+	"runtime"
 )
 
-// NewFirewall returns a nil manager
-func NewFirewall(context.Context) (firewallManager, error) {
-	return nil, fmt.Errorf("firewall not supported on this OS")
+// newFirewall returns a nil manager
+func newFirewall(context.Context) (firewallManager, error) {
+	return nil, fmt.Errorf("firewall not supported on %s", runtime.GOOS)
 }