Add property and event for debug attach (#25788)

Adds the `IsRemoteDebuggerAttached` property to `Runspace` that
indicates whether a debugger is attached to the Runspace through the
`Debug-Runspace` cmdlet. Also adds a new engine event
`PowerShell.OnDebugAttach` that is emitted when the debugger is
attached.

src/System.Management.Automation/engine/EventManager.cs

@@ -1830,6 +1830,11 @@ namespace System.Management.Automation
         /// </summary>
         public const string OnIdle = "PowerShell.OnIdle";
 
+        /// <summary>
+        /// Called when <c>Debug-Runspace</c> has attached a debugger to the current runspace.
+        /// </summary>
+        public const string OnDebugAttach = "PowerShell.OnDebugAttach";
+
         /// <summary>
         /// Called during scriptblock invocation.
         /// </summary>
@@ -1843,7 +1848,7 @@ namespace System.Management.Automation
         /// <summary>
         /// A HashSet that contains all engine event names.
         /// </summary>
-        internal static readonly HashSet<string> EngineEvents = new HashSet<string>(StringComparer.OrdinalIgnoreCase) { Exiting, OnIdle, OnScriptBlockInvoke };
+        internal static readonly HashSet<string> EngineEvents = new HashSet<string>(StringComparer.OrdinalIgnoreCase) { Exiting, OnIdle, OnDebugAttach, OnScriptBlockInvoke };
     }
 
     /// <summary>

src/System.Management.Automation/engine/hostifaces/Connection.cs

@@ -761,6 +761,12 @@ namespace System.Management.Automation.Runspaces
         /// Gets the Runspace Id.
         /// </summary>
         public int Id { get; }
+        
+        /// <summary>
+        /// Gets and sets a boolean indicating whether the runspace has a
+        /// debugger attached with <c>Debug-Runspace</c>.
+        /// </summary>
+        public bool IsRemoteDebuggerAttached { get; internal set; }
 
         /// <summary>
         /// Returns protocol version that the remote server uses for PS remoting.