replaced http client with go-ethereum client

[pompon0]

It automatically adds support for WS connections.
Additionally simplified shared sender a bit.
Renamed Workers to to TasksPerEndpoint, because it was super misleading. The json field name stays "workers" for backward compatiblity.

[cursor]

PR Summary

Medium Risk
Core load-test send path and metrics semantics change (no http_errors, RPC errors only); behavior should be equivalent for HTTP endpoints but WS and error classification need validation.

Overview
Transaction submission no longer posts hand-built JSON-RPC over HTTP; workers dial once per endpoint via newRPCClient and send with ethclient.SendTransaction, so HTTP(S) still uses the tuned otelhttp transport while WS(S) endpoints work without a separate code path. Receipt polling reuses the same client instead of dialing again in the watcher.

ShardedSender is trimmed: settings and the stats collector are passed at construction through WorkerConfig, and the post-create setter chain (SetDryRun, SetStatsCollector, etc.) is removed. Settings.Workers is renamed to TasksPerEndpoint (JSON/config key workers unchanged); ResolveSettings now returns a pointer and main keeps resolved settings on cfg.Settings.

Observability shifts with the transport change: the http_errors counter is dropped and rejections are labeled reason=rpc instead of HTTP status. Shared TPS limiting uses Limiter.Wait rather than skipping when Allow is false.

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