refactor: use task executor in payload handler

[sieniven]

📝 Summary

Use the in-built reth's task executor inside the payload handler.

✅ I have completed the following steps:

• [✅] Run make lint
• [✅] Run make test
• Added tests (if applicable)

[akundaz]

What is the purpose of this?

This also spawns flashblock execution on a non-blocking task instead of a blocking one

[sieniven]

@akundaz thanks for the spot, resolved the issue to use a blocking thread instead (cf0c020). The PR's intention is refactor and use the inbuilt reth's TaskExecutor pattern for spawning tasks, which will include reth's shutdown signals handling, tracing, metrics, etc