Allow Bulk Insert for a specified list of columns (#311)

[NTmatter]

Adds bulk_insert_columns(self, table, columns) and turns bulk_insert(self, table) into a compatibility shim that calls self.bulk_insert_columns(table, &["*"]), maintaining the existing behaviour.

This should simplify bulk inserts in cases where column order may be unpredictable, or only a subset of columns need to be inserted.