Can we change the type of the argument to `callActivity` and `callSubOrchestrator` into something other than a string?

[hossam-nasr]

Same as #418 but narrowed down scope. This issue is particularly concerned with just changing the type of the argument to context.df.callActivity() and context.df.callSubOrchestrator() into something more reliable than a string. Ideally we should do this in a non-breaking way

[hossam-nasr]

Discussed offline. Let's do this change in a non-breaking way by using overloads to allow both a string and an object returned from a df.app.activity() call.

[hossam-nasr]

Closing in favor of #418