fix: Optimizes OPL using string interpolation (#1041)

## Breaking Changes (New API)
ObjectPropertyList supports the following API:
```cs
list.Add(500000);
list.Add(500001, stringArgument);
list.Add("Some text");
list.Add($"Some text with {argument}");
list.Add(500002, $"{arg1}\t{arg2}");
```

## Notes
1. All API uses that require a formatter like this:
    ```cs
    list.Add(500002, "{0}\t{1}", arg1, arg2);
    ```
    Should be changed to use string interpolation, for example:
    ```cs
    list.Add(500002, $"{arg1}\t{arg2}");
    ```
2. The following paradigm should no longer be used:
    ```cs
    list.Add(1061170, prop.ToString()); // strength requirement ~1_val~
    ```
    The new string interpolation API will avoid having to convert the argument to a string before writing it to the packet. Instead use the following:
    ```cs
    list.Add(1061170, $"{prop}"); // strength requirement ~1_val~
    ```

### Benchmarks
```cs
|                         Method |     Mean |   Error |  StdDev |  Gen 0 | Allocated |
|------------------------------- |---------:|--------:|--------:|-------:|----------:|
|                BenchmarkOldOPL | 241.0 ns | 0.56 ns | 0.47 ns | 0.0105 |      88 B |
| BenchmarkStringInterpolatedOPL | 199.9 ns | 2.44 ns | 2.39 ns |      - |         - |
```

### Changes
- [X] Removes crash in STArray.Return when array is null.
- [X] Fixes NPE in OPL when entity is null. Serial in packet will be 0 when entity is null.
- [X] Fixes NPE in AosAttributes when Parent is null.
- [X] Changes OPL to use string interpolation.
- [X] Introduces `IPropertyList` to allow extending PropertyList for other uses.

Projects/UOContent/Engines/Spawners/BaseSpawner.cs

@@ -353,11 +353,11 @@ namespace Server.Engines.Spawners
             from.SendGump(new SpawnerGump(this));
         }
 
-        public virtual void GetSpawnerProperties(ObjectPropertyList list)
+        public virtual void GetSpawnerProperties(IPropertyList list)
         {
         }
 
-        public override void GetProperties(ObjectPropertyList list)
+        public override void GetProperties(IPropertyList list)
         {
             base.GetProperties(list);
 
@@ -365,18 +365,19 @@ namespace Server.Engines.Spawners
             {
                 list.Add(1060742); // active
 
-                list.Add(1060656, m_Count.ToString()); // amount to make: ~1_val~
-                list.Add(1061169, m_HomeRange.ToString()); // range ~1_val~
-                list.Add(1050039, "walking range:\t{0}", m_WalkingRange); // ~1_NUMBER~ ~2_ITEMNAME~
-                list.Add(1053099, "group:\t{0}", m_Group); // ~1_oretype~: ~2_armortype~
-                list.Add(1060847, "team:\t{0}", m_Team); // ~1_val~ ~2_val~
-                list.Add(1063483, "delay:\t{0} to {1}", m_MinDelay, m_MaxDelay); // ~1_MATERIAL~: ~2_ITEMNAME~
+                list.Add(1060656, $"{m_Count}");                            // amount to make: ~1_val~
+                list.Add(1061169, $"{m_HomeRange}");                        // range ~1_val~
+                list.Add(1050039, $"walking range:\t{m_WalkingRange}");     // ~1_NUMBER~ ~2_ITEMNAME~
+                list.Add(1053099, $"group:\t{m_Group}");                    // ~1_oretype~: ~2_armortype~
+                list.Add(1060847, $"team:\t{m_Team}");                      // ~1_val~ ~2_val~
+                list.Add(1063483, $"delay:\t{m_MinDelay} to {m_MaxDelay}"); // ~1_MATERIAL~: ~2_ITEMNAME~
 
                 GetSpawnerProperties(list);
 
                 for (var i = 0; i < 6 && i < Entries.Count; ++i)
                 {
-                    list.Add(1060658 + i, "\t{0}\t{1}", Entries[i].SpawnedName, CountSpawns(Entries[i]));
+                    var entry = Entries[i];
+                    list.Add(1060658 + i, $"\t{entry.SpawnedName}\t{CountSpawns(entry)}");
                 }
             }
             else
@@ -794,7 +795,7 @@ namespace Server.Engines.Spawners
             }
             else
             {
-                m_Timer?.Stop();
+                m_Timer.Stop();
                 m_Timer.Delay = delay;
             }