Allows users to pass in their own Supervisor for async steps

[Adzz]

Imagine an app called Blog that uses Sage.

Currently Sage allows async steps that spin up a task supervised under Sage.AsyncTransactionSupervisor. Because Sage is a dep of Blog, on shutdown Blog will shutdown before Sage does.

That means if an async step interacts with a process under Blog's supervision tree and you are using some sort of rolling deploy you can enter the following scenario:

1. Blog starts async task
2. Sage starts doing that task
3. Blog shuts down
4. Sage tries to interact with a process in Blog and can't - so it crashes / errors.

Possible Solutions

1. Make Sage a library and allow users to start it themselves?
2. Allow the user to pass in their own supervisor under which it can start the async steps.

This PR does 2.

[ulissesalmeida]

That's a pretty good idea. We should let caller specify which task supervisor the task should run under.

[AndrewDryga]

Hello @Adzz, I love the change.

I think the only things left are tests and adding documentation to functions explaining how we can override the default supervisor.

[Adzz]

Great stuff I'll add that now.

[Adzz]

@AndrewDryga I have added a test and some docs, please let me know if you have any suggestions.

[AndrewDryga]

@Adzz don't worry about docs formatting, LGTM. I'll slightly edit them and release a new version. Thank you ❤️

[Adzz]

Sounds good thanks for the quick feedback!