datalake: remove redundant copy of stats bounds

iobuf_to_bytes() takes its argument by const ref and copies into a fresh
bytes, so copying the bound first was an extra allocation and memcpy per
column on every file finish.

Author: andrwng

src/v/datalake/serde_parquet_writer.cc

@@ -127,10 +127,10 @@ ss::future<writer_error> serde_parquet_writer::finish() {
             ps.null_value_count = *cs.bounds.null_count;
         }
         if (cs.bounds.min) {
-            ps.lower_bound = iobuf_to_bytes(cs.bounds.min->value.copy());
+            ps.lower_bound = iobuf_to_bytes(cs.bounds.min->value);
         }
         if (cs.bounds.max) {
-            ps.upper_bound = iobuf_to_bytes(cs.bounds.max->value.copy());
+            ps.upper_bound = iobuf_to_bytes(cs.bounds.max->value);
         }
         _column_stats.push_back(std::move(ps));
     }