util: Add DownloadFile function

[ahmad-abuziad]

Enhancement

• automatically create directories if don't exist, now it fails if any directory doesn't exist in the filePath
• if filePath is empty string, create a file with random name under tmp directory and return a string(path to the file)
• parameter named to filePath with capital P to avoid confusion with the filepath package

[ahmad-abuziad]

@purpleidea please review