perf(kubernetes): optimize AutoPath slice allocation (#7323)

Pre-allocate slice capacity in AutoPath to eliminate unnecessary
memory reallocations. This avoids slice growth when appending
search domains and sentinel value.

Benchmark shows significant performance improvement:
- Before: 538.6 ns/op, 560 B/op, 13 allocs/op
- After:  436.8 ns/op, 336 B/op, 11 allocs/op
- Result: 19% faster, 40% less memory, 15% fewer allocations

The optimization benefits Kubernetes clusters using autopath for
server-side search path completion.

Adds benchmark test to measure AutoPath performance.

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

plugin/kubernetes/autopath.go

@@ -33,15 +33,12 @@ func (k *Kubernetes) AutoPath(state request.Request) []string {
 		return nil
 	}
 
-	search := make([]string, 3)
+	totalSize := 3 + len(k.autoPathSearch) + 1 // +1 for sentinel
+	search := make([]string, 0, totalSize)
 	if zone == "." {
-		search[0] = pod.Namespace + ".svc."
-		search[1] = "svc."
-		search[2] = "."
+		search = append(search, pod.Namespace+".svc.", "svc.", ".")
 	} else {
-		search[0] = pod.Namespace + ".svc." + zone
-		search[1] = "svc." + zone
-		search[2] = zone
+		search = append(search, pod.Namespace+".svc."+zone, "svc."+zone, zone)
 	}
 
 	search = append(search, k.autoPathSearch...)