[Feature] database helpers functions don't support custom create_engine()

[AlonMorgen]

functions like "database_exists()" uses sa.create_engine(url) which is the most common way, but in some cases you want to pass arguments to the create_engine function, such as creator function that uses another mechanism to get a database connection.

It would be nice if the library could support having an "engine" from outside, and not generate it inside the function. this way we can use customized engines and not just use the sa.create_engine(url) that is generated for us by the utility.