Update to allow custom engine parameter

[guiscaranse]

This will allow users to pass a custom engine when in need to patch or customize the original engine class.

[mfreeborn]

Thanks for the pull request. What would be the use case for allowing people to pass their own custom engine instead of supplying a set of arguments as a dict with engine_args, which is already possible?

[guiscaranse]

Some event listeners require an engine object, you can't do that if you use this middleware because engine object get's stored inside of the middleware class. Example: https://docs.sqlalchemy.org/en/13/core/pooling.html#using-connection-pools-with-multiprocessing

EDIT: Just to be clear, this PR has more to do with being able to access the engine object instead of customizing it

[mfreeborn]

Sounds good to me, merged it in.