kubernetes_management: add benchmark + base cluster management plane methods

kubernetes_management_benchmark.py:
- Full implementation of Scenarios A, B, C on top of Zach's skeleton
- _CleanStartSweep: guard clause, let it fail (no broad except)
- Run(): guard clause for version resolution
- All helpers: _RunScenarioA/B/C, _TimedAsync, _RunAsync, _OpSamples
- _Results accumulator, _AggregateSamples, _OutlierSamples

kubernetes_cluster.py:
- AddNodepool: delegates to CreateNodePool for standard clusters
- CreateNodePool/DeleteNodePool/UpgradeNodePool/UpdateCluster: sync
  wrappers calling *Async + WaitForOperation
- Abstract methods: CreateNodePoolAsync, DeleteNodePoolAsync,
  UpgradeNodePoolAsync, UpdateClusterAsync, WaitForOperation,
  ResolveNodePoolVersions, GetNodePoolNames
- BareMinor, AdjacentMinorBelow: version helper functions

Tested:
- 89 unit tests passing
- EKS end-to-end: 99 pools, 100% success all 7 scenarios
- pyink + lint-diffs clean

Author: Ashish Suneja

CHANGES.next.md

@@ -1,3 +1,10 @@
+### New features:
+-   Add kubernetes_management benchmark for measuring GKE/EKS/AKS management
+    plane API responsiveness. (from @ashishsuneja)
+-   Add KubernetesCluster base class management plane abstract methods:
+    CreateNodePool, DeleteNodePool, UpgradeNodePool, UpdateCluster and
+    their async counterparts. (from @ashishsuneja)
+
 ### Breaking changes:
 
 -   Added --accept_licenses flag. User have to turn this flag on to acknowledge