NetBird SSH (#361)

This PR adds support for SSH access through the NetBird network
without managing SSH skeys.
NetBird client app has an embedded SSH server (Linux/Mac only) 
and a netbird ssh command.

client/ssh/server_mock.go

@@ -0,0 +1,44 @@
+package ssh
+
+import "context"
+
+// MockServer mocks ssh.Server
+type MockServer struct {
+	Ctx                     context.Context
+	StopFunc                func() error
+	StartFunc               func() error
+	AddAuthorizedKeyFunc    func(peer, newKey string) error
+	RemoveAuthorizedKeyFunc func(peer string)
+}
+
+// RemoveAuthorizedKey removes SSH key of a given peer from the authorized keys
+func (srv *MockServer) RemoveAuthorizedKey(peer string) {
+	if srv.RemoveAuthorizedKeyFunc == nil {
+		return
+	}
+	srv.RemoveAuthorizedKeyFunc(peer)
+}
+
+// AddAuthorizedKey add a given peer key to server authorized keys
+func (srv *MockServer) AddAuthorizedKey(peer, newKey string) error {
+	if srv.AddAuthorizedKeyFunc == nil {
+		return nil
+	}
+	return srv.AddAuthorizedKeyFunc(peer, newKey)
+}
+
+// Stop stops SSH server.
+func (srv *MockServer) Stop() error {
+	if srv.StopFunc == nil {
+		return nil
+	}
+	return srv.StopFunc()
+}
+
+// Start starts SSH server. Blocking
+func (srv *MockServer) Start() error {
+	if srv.StartFunc == nil {
+		return nil
+	}
+	return srv.StartFunc()
+}