stellar#5221: return null txmeta in api tx resource model when SKIP_T…

…XMETA

services/horizon/internal/resourceadapter/main.go

@@ -0,0 +1,16 @@
+package resourceadapter
+
+type ResourceAdapter struct {
+	SkipTxmeta bool
+}
+
+var resourceAdapterInstance ResourceAdapter
+
+// provide a singleton reference to resource adapter runtime state
+func GetResourceAdapter() ResourceAdapter {
+	return resourceAdapterInstance
+}
+
+func SetResourceAdapter(instance ResourceAdapter) {
+	resourceAdapterInstance = instance
+}