Refactor Mocktail file handling to be encapsulated by the MocktailResponse class

[AdamCampbell]

• By moving the Mocktail file parsing into the MocktailResponse class Mocktail files can be reused for purposes other than running Mocktail. Whether that be generating other types of network responses, pulling out values for unit testing, or something else entirely.
• The Mocktail repo also benefits by have better separation of concerns
• This should be a pure refactor that introduces no functional change