Resolves open-telemetry#65

- Added signature field to DownloadableFile message.
- Added a short explanation about how the signature field can be used
  for detached signatures.

…n-telemetry#63)

This is a first approach towards open-telemetry#56 

It assumes that Agent's `instance_uid` can always be updated by Server (since resolving potential issues with conflicts is important).

There's a gap with multiplexed websocket connections which perhaps could be handled by adding Agent capability flag indicating if overrides can be accepted or not

…-telemetry-main