[Backport 11.2] Create a PingFilter variant to the HST PingServlet

The PingServlet has the big disadvantage that it always needs to wait for the HST model to be loaded before it can return. However, when the initial model load takes several minutes (for example due to 100.000++ hst jcr config nodes), many requests can be blocked on the ping servlet.

Instead, we should come up with a PingFilter that can be placed before the HstFilter and directly return in case the HST did not yet load its hst config nodes, see HSTTWO-4198