Remove Tasks trait from PayloadJobGenerator and PayloadJob

[haardikk21]

📝 Summary

Solves #428

Removes Tasks trait from BlockPayloadJobGenerator and BlockPayloadJob. Replaces the singular usage of the Task Executor inside spawn_build_job with a direct call to the Tokio task spawner to replicate previous behaviour without needing the trait implementation.

💡 Motivation and Context

Makes it simpler to mock the payload job generator and payload job types without needing to involve a task executor.

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✅ I have completed the following steps:

• Run make lint
• Run make test
• Added tests (if applicable)

[haardikk21]

Closing due to redundancy with work being done on #443