Allow the usage of Services as Upstream on a global level (#7469)

It is possible to change this behavior on an ingress level, which works
well when you only have a few of them. When running several dozen
ingress and with a high change rate of running pods it makes it easier
to define this configuration on a global level.

This change is completely backwards compatible, only adding the
possibility of defining a new key in the configmap.

internal/ingress/defaults/main.go

@@ -161,4 +161,8 @@ type Backend struct {
 	// Sets the maximum temp file size when proxy-buffers capacity is exceeded.
 	// http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_max_temp_file_size
 	ProxyMaxTempFileSize string `json:"proxy-max-temp-file-size"`
+
+	// By default, the NGINX ingress controller uses a list of all endpoints (Pod IP/port) in the NGINX upstream configuration.
+	// It disables that behavior and instead uses a single upstream in NGINX, the service's Cluster IP and port.
+	ServiceUpstream bool `json:"service-upstream"`
 }