Reuse existing conflated traces files for linea_generateConflatedTracesToFileV2 requests

[jonesho]

When handling linea_generateConflatedTracesToFileV2 requests, we should check if the conflated-traces file is already existed in the file system, and only re-generate the conflated-traces file if no corresponding file is found.

This could save a lot of computing resources especially when coordinator restarts (which might re-send a great deal of linea_generateConflatedTracesToFileV2 requests to besu trace node for batches that had been requested before), also we had a Sepolia incident (940) that was due to the long duration for besu traces node to generate the conflated traces file of a specific block range, which resulted in failed response and could be avoided if this feature were in place.