refactor: unique function name validation

[hallvictoria]

Main change:

• Refactor checking for unique function names logic into higher level class.
• FunctionBuilder object is at the function level. FunctionRegister class is at the app level, so it is more suited for validation across the app as a whole.
• Now, function name dict is a property of the FunctionRegister class. There is a new method added to the FunctionRegister class for validating the function names. It loops through all the functions in the apps, adds the function name to the dict, and throws an error if there's a repeat of function names. This validation method is added as part of get_functions so the non-distinct function name error is thrown in the same spot as it was previously.

Extra change:

• removes dummy username values from kafka tests to fix security warning