Make boost::json header only in the read lib

Otherwise there are unresolved external symbols when built with
visibility=hidden

Relates-To: MINOR
Signed-off-by: Rustam Gamidov <ext-rustam.gamidov@here.com>

Author: rustam-gamidov-here

olp-cpp-sdk-dataservice-read/src/utils/BoostJsonSrc.cpp

@@ -0,0 +1,20 @@
+/*
+ * Copyright (C) 2026 HERE Europe B.V.
+ *
+ * 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.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ * License-Filename: LICENSE
+ */
+
+#include <boost/json/src.hpp>