feat(services): add IRequestMeta for extensibility

Author: jaredcnance

src/JsonApiDotNetCore/Services/IRequestMeta.cs

@@ -0,0 +1,9 @@
+using System.Collections.Generic;
+
+namespace JsonApiDotNetCore.Services
+{
+    public interface IRequestMeta
+    {
+        Dictionary<string, object> GetMeta();
+    }
+}