Custom Engine (#469)

Allow passing in a custom SQLAlchemy engine through DBOS configuration.
If an engine is passed in, DBOS **exclusively** uses it to access the
system database; it does not construct any engine of its own.

For example:
```python
import sqlalchemy as sa

engine = sa.create_engine(system_database_url)

config: DBOSConfig = {
    "name": "dbos-starter",
    "system_database_url": system_database_url,
    "system_database_engine": engine,
}
DBOS(config=config)
```

Addresses https://github.com/dbos-inc/dbos-transact-py/issues/466