Add basic API, only with system endpoints

PluralKit.API/TokenAuthService.cs

@@ -0,0 +1,31 @@
+using System;
+using System.Linq;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Http;
+
+namespace PluralKit.API
+{
+    public class TokenAuthService: IMiddleware
+    {
+        public PKSystem CurrentSystem { get; set; }
+
+        private SystemStore _systems;
+
+        public TokenAuthService(SystemStore systems)
+        {
+            _systems = systems;
+        }
+
+        public async Task InvokeAsync(HttpContext context, RequestDelegate next)
+        {
+            var token = context.Request.Headers["Authorization"].FirstOrDefault();
+            if (token != null)
+            {
+                CurrentSystem = await _systems.GetByToken(token);
+            }
+            
+            await next.Invoke(context);
+            CurrentSystem = null;
+        }
+    }
+}