Web UI links for operations and tables

[sshaplygin]

Part of #6.

Three of the Go SDK's twelve examples print yt.WebUIOperationURL(...); C++ has IOperation::GetWebInterfaceUrl(). This crate prints an operation id and leaves the reader to find it.

Why it was deliberately not built

The URL is https://<host>/<cluster>/operations/<id>, and the cluster name is not derivable from a proxy address. The choice was between asking the caller for something they would have to look up, and printing a link that might be wrong. A wrong link is worse than none.

What changed

//sys/@cluster_name exists and is readable — examples/cluster_info.rs already reads it ("locasaurus" on the local cluster). So the name can be fetched rather than guessed, which makes this buildable without asking the caller anything.

• read //sys/@cluster_name, cache it on the Client
• operation_url(id) and table_url(path)
• fall back to printing nothing — not to a guess — when the attribute is absent