Introduce @ExperimentalOkHttpApi (#8175)

* Introduce @ExperimentalOkHttpApi

I'm intending to use this to release OkHttp 5.0 before
all of the new APIs are final.

* PR feedback

okhttp/api/okhttp.api

@@ -556,6 +556,9 @@ public abstract interface class okhttp3/EventListener$Factory {
 	public abstract fun create (Lokhttp3/Call;)Lokhttp3/EventListener;
 }
 
+public abstract interface annotation class okhttp3/ExperimentalOkHttpApi : java/lang/annotation/Annotation {
+}
+
 public final class okhttp3/FormBody : okhttp3/RequestBody {
 	public static final field Companion Lokhttp3/FormBody$Companion;
 	public final fun -deprecated_size ()I

okhttp/src/main/kotlin/okhttp3/ExperimentalOkHttpApi.kt

@@ -0,0 +1,46 @@
+/*
+ * Copyright (C) 2024 Block, Inc.
+ *
+ * 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.
+ */
+package okhttp3
+
+/**
+ * Marks declarations that are experimental and subject to change without following SemVer
+ * conventions. Both binary and source-incompatible changes are possible, including complete removal
+ * of the experimental API.
+ *
+ * Do not use these APIs in modules that may be executed using a version of OkHttp different from
+ * the version the module was compiled with.
+ *
+ * Do not use these APIs in published libraries.
+ *
+ * Do not use these APIs if you aren't willing to track changes to them.
+ */
+@MustBeDocumented
+@Retention(value = AnnotationRetention.BINARY)
+@Target(
+  AnnotationTarget.CLASS,
+  AnnotationTarget.ANNOTATION_CLASS,
+  AnnotationTarget.PROPERTY,
+  AnnotationTarget.FIELD,
+  AnnotationTarget.LOCAL_VARIABLE,
+  AnnotationTarget.VALUE_PARAMETER,
+  AnnotationTarget.CONSTRUCTOR,
+  AnnotationTarget.FUNCTION,
+  AnnotationTarget.PROPERTY_GETTER,
+  AnnotationTarget.PROPERTY_SETTER,
+  AnnotationTarget.TYPEALIAS
+)
+@RequiresOptIn(level = RequiresOptIn.Level.ERROR)
+public annotation class ExperimentalOkHttpApi