Add API for bulk inserts

For backends that support it.
Similar to, for MySQL:
* `LOAD DATA INFILE`
* Bulk inserts (`INSERT INTO`): `INSERT INTO class_result VALUES (1,’John’,’A’),(2,’Elsa’,’D’),(3,’Sophia’,’B’),(4,’Paul’,’B’),(5,’Saira’,’A’)`

Start by locating the existing insert API and the backend-specific database adapters. Compare the supported backends with bulk-insert capabilities such as MySQL's LOAD DATA INFILE and multi-row INSERT syntax, then define tests showing which backends support the new API and what completion looks like.

Written by the indexing model from the issue text.