Add new-formatted List callback

[azretkenzhaliev]

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 separate callback for ../list REST endpoints.

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.

[azretkenzhaliev]

Done. Instead of doing separate callbacks for info and list we unified them under FileSearch callback.