implement server spool to disk & replay

Via some discussion in #183: It might be useful to have otel-cli be able to write spans to disk, and then replay them. This can be handy in scenarios when the network/collector aren't available when the traces are generated, but will be later.

Also creates a path for having containers with no networking write to a bind mounted path, then an external otel-cli picks them up and sends along.

spool=$(mktemp -d)

# write incoming spans to the /tmp/$spool directory
otel-cli server spool --directory /tmp/$spool --protocol http/protobuf --endpoint localhost:4318

# or, use otel-cli as usual but point endpoint to disk? this would be nice for debugging...
otel-cli exec --endpoint file://$spool -- sleep 1

# read all the spans in this directory and send them upstream
otel-cli server replay --directory /tmp/$spool --endpoint https://my-collector:9999

Seems like plain flock() should be sufficient for locking. I think I would only support protobuf to avoid type erasure. I started looking at how much work it is, and it's not too bad, and not super invasive. I did the first bits to expose the raw protobuf spans and events through CliEvent here: https://github.com/equinix-labs/otel-cli/tree/spool-spans-to-disk

@garthk did I understand your use case correctly?

Begin by reading issue #183 and the spool-spans-to-disk branch, especially the existing CliEvent exposure for raw protobuf spans and events. Validate the design against the documented spool and replay command examples, including directory locking and protobuf-only behavior; done means spans can be written offline and later replayed to the collector.