Include draining endpoints when persistent-drainable

I believe that this is the only place that needs to check for
`Serving` Endpoint objects to implement this feature.

There are 6 total invocations of `getEndpointsFromSlices` in
the repo, all in `controller.go`.

2 of them are relating to `L4Service`. I'm assuming L4 is referring
to the OSI model and these seem to relate to TCP and UDP, which
have no knowledge of HTTP cookies, which `persistent-drainable`
relies on, so these can't be required. (They appear to be for
controller level config instead of ingress rules).

A third is in `getDefaultUpstream`. My understanding is this is for
a fallback service should all of the primary services in the ingress
rules be down. This seems like a fallback service handler and not
useful for `persistent-drainable`.

A fourth one is used in `getBackendServers` when building the
`custom-default-backend-` upstream when no Endpoints are available
for the service. Again, this doesn't seem intended for the primary
application that needs session affinity, so exluding the draining
Endpoints in the default backend seems desirable.

The last two are used in `serviceEndpoints`, which itself is called
in two places, both in `createUpstreams`. One of them is for the
default backend service, which discussed above, should not be
included. The other one is for our main ingress rules and **is** the
one that we want to update.

Author: gulachek

internal/ingress/controller/controller.go

@@ -1114,8 +1114,13 @@ func (n *NGINXController) createUpstreams(data []*ingress.Ingress, du *ingress.B
 				}
 
 				if len(upstreams[name].Endpoints) == 0 {
+					epSelectionMode := ReadyEndpoints
+					if anns.SessionAffinity.Mode == "persistent-drainable" {
+						epSelectionMode = ServingEndpoints
+					}
+
 					_, port := upstreamServiceNameAndPort(path.Backend.Service)
-					endp, err := n.serviceEndpoints(svcKey, port.String(), ReadyEndpoints)
+					endp, err := n.serviceEndpoints(svcKey, port.String(), epSelectionMode)
 
 					if err != nil {
 						klog.Warningf("Error obtaining Endpoints for Service %q: %v", svcKey, err)