Add inject_args to spies

[awestendorf]

There are cases where one wants to use a spy as a means to hook into the call stack and change a parameter to alter the behavior of code further down the stack. One option would be to attach a side_effect to a previous spy or expectation, and use that side effect to alter the state of the module to be called on a subsequent execution of a spy. However, that's not always feasible and can be challenging to understand when reading a test.

This proposal is for a new method, inject_args, that would allow the user to hook into the spy and change the arguments. Probably the easiest approach is for this to accept a callable parameter, and give the user complete control over how to transform the original *args, **kwargs into what's needed for the test.