GH4066/4324: RunTargets opt-in unified dependency graph

[devlead]

• Add opt-in unified dependency graph for multiple targets so shared dependencies run once; legacy per-target traversal kept as default, or controlled via or boolean config (Settings_UnifiedDependencyGraphForMultipleTargets)
• Add ExecutionSettings.UnifiedDependencyGraphForMultipleTargets, enable via UseUnifiedDependencyGraphForMultipleTargets()
• Add CakeGraph.Traverse(IEnumerable) and TopologicalSort for merged dependency order
• Add ICakeConfiguration.GetBoolValue() to reduce code duplication and use it for boolean config in BuildScriptHost, ProcessRunner, ScriptProcessor, AssemblyVerifier
• Add unit tests for unified/legacy RunTargets and SetupContext.TasksToExecute; add CakeGraph multi-target traverse tests
• Add integration tests (targets.cake) for unified vs legacy behavior, using --expected and env; add shared dependency F for A,B,C
• fixes SetupContext.TasksToExecute only lists tasks related to first target when calling RunTargets #4066
• fixes When running multiple targets, common dependent tasks are executed twice #4324

Before change

────────────────────────────────────────────────────────────────────────────────────────────────────
 Setup
────────────────────────────────────────────────────────────────────────────────────────────────────
Tasks to run: F, A

.....

  Task                            Duration                                   Status
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
  Setup                           00:00:00.0649513                           Succeeded
  F                               00:00:00.0075351                           Succeeded
  A                               00:00:00.0008039                           Succeeded
  F                               00:00:00.0005486                           Succeeded
  B                               00:00:00.0007980                           Succeeded
  F                               00:00:00.0004626                           Succeeded
  C                               00:00:00.0004866                           Succeeded
  Teardown                        00:00:00.0026602                           Succeeded
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
  Total:                          00:00:00.0782463

After

────────────────────────────────────────────────────────────────────────────────────────────────────
 Setup
────────────────────────────────────────────────────────────────────────────────────────────────────
Tasks to run: F, A, B, C

.....


  Task                            Duration                                   Status
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
  Setup                           00:00:00.0764160                           Succeeded
  F                               00:00:00.0157535                           Succeeded
  A                               00:00:00.0015117                           Succeeded
  B                               00:00:00.0005766                           Succeeded
  C                               00:00:00.0005362                           Succeeded
  Teardown                        00:00:00.0017801                           Succeeded
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━