add transaction data context manipulation

Author: giortzisg

docs/platforms/go/common/tracing/instrumentation/custom-instrumentation/index.mdx

@@ -95,6 +95,13 @@ sentry.Init(sentry.ClientOptions{
       sp.SetData("dataAttr1", 42)
       sp.SetData("dataAttr2", true)
     }
+
+    dataCtx, ok := event.Contexts["trace"]["data"].(map[string]any)
+    if !ok {
+      dataCtx = make(map[string]any)
+      event.Contexts["trace"]["data"] = dataCtx
+    }
+    dataCtx["num"] = 42
     return event
   },
 })