Also requested by Paul Millar
Basically, let the client application (e.g. FTS3) specify a set of key/value pairs that would be sent to the endpoints.
For instance, job id, file id, and retry (but anyway, that's up to FTS3).
For GridFTP globus_ftp_client_handleattr_set_clientinfo can be used.
For XROOTD, FileSystem::SendInfo looks like it.
For HTTP, just tell Davix to pass a custom header (ClientInfo)
For SRM (soap), also a custom header can be used, but looking at the source code it seems that the default post hook (http_post) doesn't let room for custom headers, so we may need to wrap it.