Add new-formatted retriever function

[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 retriever function.

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]

Retriever function will take the following arguments:

1. fileID string
2. name string
3. owner string
4. token string
5. tags string
6. version string
7. limit int
8. offset int
9. repo string

[azretkenzhaliev]

Retrieve must return SearchResult structure that will have the following arguments:

1. fileID (MyBucket -> fileID)
2. owner (MyBucket -> fileID -> owner -> first key/value pair)
3. name (MyBucket -> fileID -> key:name)
4. repo (MyBucket -> fileID -> type -> buckets)
5. version (MyBucket -> fileID -> key:version)
6. scope (MyBucket -> fileID -> scope -> all key/value pairs)
7. md5 (MyBucket -> fileID -> hash -> key:md5)
8. sha256 (MyBucket -> fileID -> hash -> key:sha256)
9. size (MyBucket -> fileID -> size)
10. tag (MyBucket -> fileID -> tag)

[azretkenzhaliev]

Done.