Test cases for Custom URL signer

[anthony-chaudhary]

• Unit tests for simple formatting items, e.g. string signed format
  Related to url_generation needs refactor and classes #1525 but focused on the actual test for it regardless of refactor

• Scope out End to end test actually mocking and generating a custom signer
  Scope not make because in part: Custom URL signer is a critical feature for some people but need to think a bit more about what general case is / what level we want to support this