Allow `exec_with_returning_many` with MariaDB

Motivation

Currently, when using a MariaDB as backend, it is hidden behind the MySql backend, which is normal. However, there are things that are not available in MySQL but in MariaDB, like the INSERT ... RETURNING syntax. Nevertheless, the methods building queries with the "returning" syntax are configured to be only available with the Postgres and Sqlite backend (see https://docs.rs/sea-orm/latest/src/sea_orm/database/db_connection.rs.html#585).

Proposed Solutions

Just add a match arm in the DbBackend::support_returning method, for the MySql backend, with an additional feature, similar to "sqlite-use-returning-for-3_35", e.g. "mariadb".

This way, developers using MariaDB will be able to use the "returning" syntax, by enabling this feature.