Support non-function activity callables

[cretz]

What was changed

Tests and minor changes to support non-function callables as activities (e.g. classes with __call__)

Checklist

1. Closes [Feature Request] Support non-function callables as activities #52

[cretz]

We have decided to revisit this to make class and method activity callables better supported. Current thoughts:

• See if we can accept callable classes (i.e. classes with __call__) directly in execute_activity, e.g. workflow.execute_activity(MyActivityClass). Otherwise we will add execute_activity_class (and all start/local variants).
• Due to likely ambiguity, for methods we will likely have execute_activity_method (and all start/local variants), e.g. workflow.execute_activity_method(MyClass.my_method).

I am marking this PR as draft while this is developed.

[cretz]

Closing this. There are a lot of typing changes coming to support this. Opening new PR...