Client: (WIP) rework CLI startup logic using RpcConfig (#3983)

[ether-wan]

This PR starts addressing #3983 by refactoring the client CLI

Description

• Introduced a new RpcConfig class for RPC endpoints (eth/engine, http/ws)
• Added a generateRpcConfigs() func to generate all server configurations from CLI args
• Refactored startRPCServers() to use these configs instead of cli args
• Removed repeated logic for creating, starting and logging HTTP/WS servers
• Split the run() function to improve testability:
  • run() now handles only CLI argument parsing, config generation, and dispatch
  • New startClientAndServers() function performs the client and RPC server startup
  • This split improves separation of concerns and makes the CLI flow easier to test

This is still a work in progress — if the current structure looks good, I’ll follow up with unit tests for the new helpers and config logic in a separate commit!