Add new-formatted Download callback

[baytulakova]

For refactoring purposes, we need to make a server application that will exist along with the current CDN application. Long in short, it will look like

• /
  • apt/
  • auth/
  • ...
  • server/
    • main.go
    • retriever.go
    • callbacks.go
  • ...
    We need to add a callback for Download function to handle upload queries and start testing our refactor.

The callback will build a query with a special method and pass this query to the retriever function. Retriever function will do its search using the search function and return a slice of files to the callback function.