[Proposal] Add `savepoint()` method to PDO

Hi internals, I'm thinking of adding a savepoint(string $name) method to PDO. Similarly, add releaseSavepoint(string $name) and change the signature of rollback() to rollback(?string $name = null). Savepoint is a feature defined in SQL99, so it is a feature supported by many databases. Therefore, I thought it would be a good idea to support this with PDO. If we use savepoint() before a transaction has started, it is expected that a PDOException will be thrown. Similarly, if we use releaseSavepoint() or rollback('savepoint_name’) when there is no savepoint, an exception will be thrown. Only pdo_odbc cannot predict what kind of database will be used, but in the odbc3 API, we can use supportsSavepoints() to find out whether the database supports savepoints. FYI, mysqli already has support for savepoints. I would like to know your opinion. Regard. Saki