feature: build overrides (#2212)

* add build overrides

* override docs

* add server submodule

* add overrides to build step

* remove testing api url

Co-Authored-By: Quahu <quahuu@gmail.com>

Co-authored-by: Quahu <quahuu@gmail.com>

experiment/Discord.Net.BuildOverrides/IOverride.cs

@@ -0,0 +1,34 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Reflection;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Discord.Overrides
+{
+    /// <summary>
+    ///     Represents a generic build override for Discord.Net
+    /// </summary>
+    public interface IOverride
+    {
+        /// <summary>
+        ///     Initializes the override.
+        /// </summary>
+        /// <remarks>
+        ///     This method is called by the <see cref="BuildOverrides"/> class
+        ///     and should not be called externally from it.
+        /// </remarks>
+        /// <param name="context">Context used by an override to initialize.</param>
+        /// <returns>
+        ///     A task representing the asynchronous initialization operation.
+        /// </returns>
+        Task InitializeAsync(OverrideContext context);
+
+        /// <summary>
+        ///     Registers a callback to load a dependency for this override.
+        /// </summary>
+        /// <param name="func">The callback to load an external dependency.</param>
+        void RegisterPackageLookupHandler(Func<string, Task<Assembly>> func);
+    }
+}