Allow to send files

[jordeu]

Current implementation when Tower request a file from the Agent a cat <filename> it is executed and the response is returned as a result. This works for small files, but for big files or binary files it may be problematic.

The goal of this issue is to implement a new type of request where Tower requests a specific file and the Agent reads it and sends it back to Tower. It has to support the streaming of big files and binary files of any format.

NOTE: Given the current limit of 5MB per websocket message, this will require a paging mechanism to send big files using several messages.