Add should be called in init func of an implementing Function.

GetFunction looks up a function by name and then init's it with the provided map.

RegisteredFunctions returns a slice of the names of every function registered.

ErrNotFound gives the details of the failed function.

Mock can be used in tests to track whether Function.Apply was called and test for expected errors.

Function has a single defined function to serve the purpose of apply logic to a message in order to return a message.

Creator defines the init structure for a Function.