Add flashblocks index to payload building context

[avalonche]

📝 Summary

Adds the flashblocks index and target flashblocks to the payload building context.

💡 Motivation and Context

The flashblocks index is needed in the build context to determine which flashblocks requires which builder tx depending on the flashblock index.

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✅ I have completed the following steps:

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