Expose sqlalchemy engine in global DBOS class

[nnayr]

It would be useful to export the sqlalchemy engine initialized here: https://github.com/dbos-inc/dbos-transact-py/blob/main/dbos/_app_db.py#L63 in the DBOS global instance. This would prevent needing to initialize a new engine to perform database operations outside of @DBOS.transaction steps.

[kraftp]

Got it! Just to be clear, while that's possible, that engine uses the default (synchronous) driver, so this doesn't resolve #190. Also, one useful method for creating your own engine is get_dbos_database_url.

[nnayr]

Yeah, I used that helper function to initialize a new engine. I'm not well versed in python and the state of its asyncio. Just thought this could be a useful feature since each engine is costly with its own connection pool.