feat(rust)!: make add_columns a builder (#3778)

Table::add_columns now takes no arguments and returns AddColumnsBuilder,
so calls become .add_columns().transform(t).execute().

read_columns was the second positional argument but reaches only one of
the five transform variants. In lance's add_columns_to_fragments only
BatchUDF receives the caller's value: SqlExpressions replaces it with
the columns its expressions reference, Stream and Reader pass None, and
AllNulls reads nothing. So it was mandatory on every call -- all
eighteen call sites here passed None -- and silently discarded four
times out of five. As a builder method it is optional, and setting it
where lance would discard it is now an error, which does reject a call
that previously succeeded while ignoring the argument.

Matches the builders add, update, and merge_insert already use.

python/src/table.rs

@@ -1375,7 +1375,12 @@ impl Table {
 
         let inner = self_.inner_ref()?.clone();
         future_into_py(self_.py(), async move {
-            let result = inner.add_columns(definitions, None).await.infer_error()?;
+            let result = inner
+                .add_columns()
+                .transform(definitions)
+                .execute()
+                .await
+                .infer_error()?;
             Ok(AddColumnsResult::from(result))
         })
     }
@@ -1389,7 +1394,12 @@ impl Table {
 
         let inner = self_.inner_ref()?.clone();
         future_into_py(self_.py(), async move {
-            let result = inner.add_columns(transform, None).await.infer_error()?;
+            let result = inner
+                .add_columns()
+                .transform(transform)
+                .execute()
+                .await
+                .infer_error()?;
             Ok(AddColumnsResult::from(result))
         })
     }