Add support for Dependency Injection with IMemoryDb (#520)

## Motivation and Context (Why the change? What's the scenario?)

Allow to inject IMemoryDb types, without the need to build an instance.

Related to #503, it
allows to register an `IMemoryDb` using Dependency Injection.

service/Abstractions/KernelMemoryBuilderExtensions.cs

@@ -165,6 +165,13 @@ public static IKernelMemoryBuilder WithCustomMemoryDb(
         return builder;
     }
 
+    public static IKernelMemoryBuilder WithCustomMemoryDb<T>(
+        this IKernelMemoryBuilder builder) where T : class, IMemoryDb
+    {
+        builder.AddSingleton<IMemoryDb, T>();
+        return builder;
+    }
+
     public static IKernelMemoryBuilder WithCustomTextGenerator(
         this IKernelMemoryBuilder builder,
         ITextGenerator service)