Add S3/ObjectStore write support for DuckLake catalogs

[shefeek-jinnah]

Summary

• Convert write pipeline from std::fs to object_store::put() with buffer approach (ArrowWriter<Vec<u8>>)
• Replace PathBuf with String throughout the write config chain (catalog, schema, table)
• DuckLakeTableWriter::new() now takes Arc<dyn ObjectStore> as second parameter
• finish(), write_table(), append_table() are now async
• DuckLakeInsertExec obtains ObjectStore from RuntimeEnv (same pattern as the read path)
• Dropped sessions are safe — buffer is discarded without uploading