Add purge commands for streams and topics; make create IDs optional

Add purge commands for streams and topics; make create IDs optional: a task in LegoFlow-SWE (Harbor dataset). The iggy CLI currently lacks purge subcommands for both stream and topic groups. Users have no way to remove all messages from a stream or a topic via the command line. Additionally, the…

The task

The `iggy` CLI currently lacks `purge` subcommands for both `stream` and `topic` groups. Users have no way to remove all messages from a stream or a topic via the command line. Additionally, the `create` commands for streams and topics force the user to supply a numeric ID instead of allowing the server to…

Part of Lego-X/LegoFlow-SWE.