Add budget backend

[rtghan]

added the application business rules layer for the "add budget" use case, but I haven't done any of the front end stuff yet since that will require more coordination between the use cases.

also, I changed the Budget class to include a Month attribute, so that we can properly store expenses with the month/budget they belong to. I know we mentioned having using custom timeframes, but I think for starters, we can just use a month.

[ErnestYuen]

November 11, 2023 Code Review: The naming conventions are clear and concise, accurately depicting what each class and method does. The access modifiers make sense and there are appropriate getters and setters for classes with private instance attributes. It may be better to move the DAO into a separate package later but for now the layers of clean architecture are well represented by current packaging. The code adheres well to the clean architecture model. The description of the pull request is clear for what has been committed. However, there is a lack of any javadocs or testing for the classes and methods.