Add ContainerViewController init with AnyWorkflowConvertable

[dhavalshreyas]

Currently, we don't have an API to create a ContainerViewController with AnyWorkflow type. This adds an extension to make that possible.

This is mostly a copy-paste from code @bencochran shared with me privately.

[JustinDSN]

Should we add a unit test to make sure this works as expected, and stays working?

[bencochran]

Should we add a unit test to make sure this works as expected, and stays working?

Yeah, that’d probably be a good idea, especially since this relies on Swift knowing that the call to self.init(workflow:… is the other (more-specifically-constrained) initializer and not a recursive call to self.

[dhavalshreyas]

Should we add a unit test to make sure this works as expected, and stays working?

Done!

[bencochran]

Test looks good, thanks!