fix: add missing direct includes in file_writer.h

file_writer.h uses std::string, std::unordered_map, and std::vector
directly but relies on transitive includes for them. Add the missing
direct includes to follow the include-what-you-use principle.

Author: shangxinli

src/iceberg/file_writer.h

@@ -25,6 +25,9 @@
 #include <functional>
 #include <memory>
 #include <optional>
+#include <string>
+#include <unordered_map>
+#include <vector>
 
 #include "iceberg/arrow_c_data.h"
 #include "iceberg/file_format.h"