Add workaround for Swift header

FirebaseInAppMessaging/Swift/Source/SPMSwiftHeaderWorkaround.swift

@@ -0,0 +1,30 @@
+// Copyright 2023 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#if SWIFT_PACKAGE
+  @_exported import FirebaseInAppMessagingInternal
+
+  // This is a trick to force generate a `FirebaseInAppMessaging-Swift.h`
+  // header that re-exports `FirebaseInAppMessagingInternal` for Objective-C
+  // clients. It is important for the below code to reference a Remote
+  // Config symbol defined in Objective-C as that will import the symbol's
+  // module (`FirebaseInAppMessagingInternal`) in the generated header. This
+  // allows Objective-C clients to import Remote Config's Objective-C API
+  // using `@import FirebaseInAppMessaging;`. This API is not needed for Swift
+  // clients and is therefore unavailable in a Swift context.
+  @available(*, unavailable)
+  @objc public extension InAppMessaging {
+    static var __no_op: () -> Void { {} }
+  }
+#endif // SWIFT_PACKAGE