CMake: require Parquet when using system Arrow

When using system Arrow, ensure Parquet is also discovered so
Parquet::parquet_* targets are available for iceberg_bundle
linking. This restores find_package(Parquet) and adds Parquet
to ICEBERG_SYSTEM_DEPENDENCIES to avoid missing target errors.

Author: MisterRaindrop

cmake_modules/IcebergThirdpartyToolchain.cmake

@@ -151,7 +151,9 @@ function(resolve_arrow_dependency)
     endif()
   else()
     set(ARROW_VENDORED FALSE)
-    list(APPEND ICEBERG_SYSTEM_DEPENDENCIES Arrow)
+    find_package(Arrow CONFIG REQUIRED)
+    find_package(Parquet CONFIG REQUIRED)
+    list(APPEND ICEBERG_SYSTEM_DEPENDENCIES Arrow Parquet)
   endif()
 
   set(ICEBERG_SYSTEM_DEPENDENCIES