Add New PodLifeTime Strategy

The new PodLifeTime descheduler strategy can be used to evict pods that
were created more than the configured number of seconds ago.

In the below example pods created more than 24 hours ago will be evicted.
````
apiVersion: "descheduler/v1alpha1"
kind: "DeschedulerPolicy"
strategies:
  "PodLifeTime":
     enabled: true
     params:
        maxPodLifeTimeSeconds: 86400
````

pkg/api/types.go

@@ -17,7 +17,7 @@ limitations under the License.
 package api
 
 import (
-	"k8s.io/api/core/v1"
+	v1 "k8s.io/api/core/v1"
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
 )
 
@@ -49,6 +49,7 @@ type StrategyParameters struct {
 	NodeResourceUtilizationThresholds *NodeResourceUtilizationThresholds
 	NodeAffinityType                  []string
 	PodsHavingTooManyRestarts         *PodsHavingTooManyRestarts
+	MaxPodLifeTimeSeconds             *uint
 }
 
 type Percentage float64