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/firewall/firewall.go

@@ -40,6 +40,9 @@ const (
 // It declares methods which handle actions required by the
 // Netbird client for ACL and routing functionality
 type Manager interface {
+	// AllowNetbird allows netbird interface traffic
+	AllowNetbird() error
+
 	// AddFiltering rule to the firewall
 	//
 	// If comment argument is empty firewall manager should set