STELLAR-3555 : add flag to enable writing telemetry data to file

[davidk81]

• adds new flag -o or --output-file to enable file writing
• will either create or append file
• cli will exit if create/append fails
• cli will exit on write errors
• uses buffered writer

integration test indicate output file is 0-padded at the end. i think this is an issue with upstream datasource.

[davidk81]

@danada tested on alpha with fake-sr. netcat was missing one message b/c i couldn't get fake-sr in the cloud to work with triggered streams. i think it should be ok to merge this.

[danada]

I'll give it a shot with a real starpass.