MM-43918: freemium limit for storage (#20225)

Summary
Adds an API end point to return storage usage

Ticket Link
https://mattermost.atlassian.net/browse/MM-43918

api4/usage_test.go

@@ -50,6 +50,20 @@ func TestGetPostsUsage(t *testing.T) {
 	})
 }
 
+func TestGetStorageUsage(t *testing.T) {
+	t.Run("unauthenticated users cannot access", func(t *testing.T) {
+		th := Setup(t)
+		defer th.TearDown()
+
+		th.Client.Logout()
+
+		usage, r, err := th.Client.GetStorageUsage()
+		assert.Error(t, err)
+		assert.Nil(t, usage)
+		assert.Equal(t, http.StatusUnauthorized, r.StatusCode)
+	})
+}
+
 func TestGetTeamsUsage(t *testing.T) {
 	t.Run("unauthenticated users can not access", func(t *testing.T) {
 		th := Setup(t)