add configurable HUD crosshair

resources/config/crosshair_config.json

@@ -0,0 +1,21 @@
+{
+  "enabled": true,
+
+  "referenceResolution": [1280, 720],
+
+  "color": [1.0, 1.0, 1.0],
+  "cl_crosshairalpha": 1.0,
+  "cl_crosshairthickness": 2.0,
+
+  "centerGapPx": 10.0,
+
+  "top": {
+    "lengthPx": 14.0,
+    "angleDeg": 90.0
+  },
+
+  "arms": [
+    { "lengthPx": 20.0, "angleDeg": 210.0 },
+    { "lengthPx": 20.0, "angleDeg": 330.0 }
+  ]
+}