Expose sqlx::PoolConnection in DatabaseTransaction

[halldorrt]

Idea

It would be nice to have access to the sqlx PoolConnection in the DatabaseTransaction struct for teams that combine raw sql and sea-orm. This would be similar to DatabaseConnection.get_postgres_connection_pool().

I know that sea-orm supports raw sql, but there are some features in sqlx that are quite useful, like compile time query checking that sea-orm doesn't support. Sqlx is also centered on sql, so with respect, it might be a bit more flexible and easy to use for sql.

This would make it very easy to use sea-orm and sqlx together in the whole stack, and it would help my team quite a lot.

I do realize that it might cause problems to expose the PoolConnection itself, in case someone e.g. commits the transaction outside of the sea-orm DatabaseTransaction. Perhaps there is a way to return the Executor trait from sqlx to mitigate this.

We'd be happy to contribute if the feedback on the idea is positive.