[FEATURE] Remove execution from FugueWorkflow context manager, remove engine from FugueWorkflow

[goodwanghan]

Currently, we can use FugueWorkflow + execution engine in this way:

with FugueWorkflow(engine) as dag:
    dag.df(..).show()

This is not an ideal design. Workflow definition shouldn't be related with execution engine. So in this change, we no longer allow FugueWorkflow to take engine as input, and the with statement becomes a pure cosmetic syntax to make boundary of the dag definition. Instead, you can always do:

dag = FugueWorkflow({"compile_configs":"..."}):
dag.df(..).show()

dag.run(engine)

It's a bit more coding, but it separates compile time and run time definitions.