Added missing license header

Author: sjahreis

kotlin-http-client/kotlin-http-client-api/src/main/kotlin/com/linkedplanet/kotlinhttpclient/api/http/HttpClient.kt

@@ -1,3 +1,22 @@
+/*-
+ * #%L
+ * kotlin-http-client-api
+ * %%
+ * Copyright (C) 2022 - 2023 linked-planet GmbH
+ * %%
+ * 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.
+ * #L%
+ */
 package com.linkedplanet.kotlinhttpclient.api.http
 
 import arrow.core.Either