Add utility AI (#806)

Co-authored-by: Pieter-Jan Briers <pieterjan.briers@gmail.com>
Co-authored-by: Metal Gear Sloth <metalgearsloth@gmail.com>
Co-authored-by: Pieter-Jan Briers <pieterjan.briers+git@gmail.com>

Content.Server/AI/Operators/Sequences/SequenceOperator.cs

@@ -0,0 +1,44 @@
+using System;
+using System.Collections.Generic;
+
+namespace Content.Server.AI.Operators.Sequences
+{
+    /// <summary>
+    /// Sequential chain of operators
+    /// Saves having to duplicate stuff like MoveTo and PickUp everywhere
+    /// </summary>
+    public abstract class SequenceOperator : AiOperator
+    {
+        public Queue<AiOperator> Sequence { get; protected set; }
+
+        public override Outcome Execute(float frameTime)
+        {
+            if (Sequence.Count == 0)
+            {
+                return Outcome.Success;
+            }
+            
+            var op = Sequence.Peek();
+            op.TryStartup();
+            var outcome = op.Execute(frameTime);
+
+            switch (outcome)
+            {
+                case Outcome.Success:
+                    op.Shutdown(outcome);
+                    // Not over until all operators are done
+                    Sequence.Dequeue();
+                    return Outcome.Continuing;
+                case Outcome.Continuing:
+                    return Outcome.Continuing;
+                case Outcome.Failed:
+                    op.Shutdown(outcome);
+                    Sequence.Clear();
+                    return Outcome.Failed;
+                
+                default:
+                    throw new ArgumentOutOfRangeException();
+            }
+        }
+    }
+}