statement-store: add statement-ops-bench for per-node RPC benchmarks

[alexggh]

A new statement-ops-bench binary alongside the existing statement-latency-bench in substrate/client/statement-store/statement-latency-bench/. While the latter measures aggregate ring-topology latency, statement-ops-bench targets individual statement-store RPC operations on specific nodes via four subcommands:

• submit — measures statement_submit duration per endpoint, with optional --iteration-batch to pipeline submits in parallel on a single ws connection.
• propagation — measures submit→subscribe latency across every (submit, subscribe) pair, with separate ws connections per side.
• subscribe — measures per-node read latency via statement_subscribeStatement with a topic filter. With --topic, the bench deliberately assumes statement already exists in the store and reads time out cleanly if no matching statement is present.
• loop — periodically run the above; stops on --iterations, --duration-secs, or Ctrl-C.