refactor(swap_encryption/pr1): correct PKB structure - swap_config as NodepoolSpec field

BREAKING: replaces SwapNodePool (standalone nodepool lifecycle) with the
correct PKB pattern: swap configuration declared in BENCHMARK_CONFIG and
applied by the existing GKE cluster creation flow.

New files:
- resources/container_service/swap_config.py
  - GkeSwapConfig(BaseResource): WriteLinuxConfigYaml(), ValidHyperdiskThroughput()
  - EksSwapConfig(BaseResource): stub for nodeadm config (deferred to PR #6780)

Core framework changes:
- configs/container_spec.py: add SwapConfigSpec(BaseSpec) + _SwapConfigDecoder
  + swap_config field on NodepoolSpec
- resources/container_service/container.py: add swap_config attr to BaseNodePoolConfig
- resources/container_service/container_cluster.py: propagate swap_config in
  _InitializeNodePool() (mirrors sandbox_config pattern)
- providers/gcp/google_kubernetes_engine.py: _AddNodeParamsToCmd() reads
  nodepool_config.swap_config - applies --system-config-from-file,
  UBUNTU_CONTAINERD, --no-enable-autorepair, boot-disk-provisioned-iops/throughput

Thin benchmark:
- BENCHMARK_CONFIG declares benchmark nodepool with swap_config (no separate
  nodepool create needed - GKE cluster creation handles it)
- Prepare(): deploy SwapDaemonSet + delete default-pool
- Run(): verify swap_active + swap_encrypted; report samples
- Cleanup(): empty (PKB auto-deletes spec.resources)

Addresses Ajay reviews:
- r3457826290: swap as base resource plugged into GKE cluster creation flow
- r3457877984: linuxConfig.swapConfig via --system-config-from-file (GkeSwapConfig)
- r3457928855: removed memory.swap.max hack
- r3457964593: UBUNTU_CONTAINERD set per-nodepool in _AddNodeParamsToCmd
- r3472513706: swapConfig auto-enables memorySwapBehavior=LimitedSwap
- r3472549985: UBUNTU_CONTAINERD required for dm-crypt

Author: DevVegeta

perfkitbenchmarker/linux_benchmarks/swap_encryption_benchmark.py

@@ -88,9 +88,18 @@
     'Override disk type for the benchmark nodepool.',
 )
 
+_DAEMONSET_IMAGE = flags.DEFINE_string(
+    'swap_encryption_daemonset_image',
+    'ubuntu:22.04',
+    'Container image for the privileged benchmark DaemonSet.',
+)
+
 _BenchmarkSpec = benchmark_spec.BenchmarkSpec
 _BENCHMARK_NODEPOOL = 'benchmark'
 _DEFAULT_POOL = 'default-pool'
+_DS_NAME = 'pkb-swap-benchmark'
+_DS_NAMESPACE = 'default'
+_DS_LABEL = 'pkb-swap-benchmark'
 
 
 def GetConfig(user_config: dict[str, Any]) -> dict[str, Any]:
@@ -124,7 +133,13 @@ def Prepare(spec: _BenchmarkSpec) -> None:
     spec: PKB BenchmarkSpec with spec.container_cluster already created.
   """
   cluster = spec.container_cluster
-  daemonset = swap_daemonset.SwapDaemonSet(cluster=cluster)
+  daemonset = swap_daemonset.SwapDaemonSet(
+      name=_DS_NAME,
+      namespace=_DS_NAMESPACE,
+      label=_DS_LABEL,
+      nodepool=_BENCHMARK_NODEPOOL,
+      image=_DAEMONSET_IMAGE.value,
+  )
   daemonset.Create()
   spec.resources.append(daemonset)
   pod = daemonset.WaitForPod()