chore: Added Initial Engine Code

Engine.Core/Abstract/Assignable/IAssignableEntity.cs

@@ -0,0 +1,26 @@
+using System;
+
+namespace Syntriax.Engine.Core.Abstract;
+
+/// <summary>
+/// Indicates the object is an <see cref="IAssignable"/> with an assignable <see cref="IEntity"/> field.
+/// </summary>
+public interface IAssignableEntity : IAssignable
+{
+    /// <summary>
+    /// Callback triggered when the <see cref="IEntity"/> value has has been assigned a new value.
+    /// </summary>
+    Action<IAssignableEntity>? OnEntityAssigned { get; set; }
+
+    /// <inheritdoc cref="IEntity" />
+    IEntity Entity { get; }
+
+    /// <summary>
+    /// Assign a value to the <see cref="IEntity"/> field of this object
+    /// </summary>
+    /// <param name="entity">New <see cref="IEntity"/> to assign.</param>
+    /// <returns> 
+    /// <see cref="true"/>, if the value given assigned successfully assigned, <see cref="false"/> if not.
+    /// </returns>
+    bool Assign(IEntity entity);
+}