refactor: cost estimator

[ratankaliani]

Refactor the flow for how the cost estimator computes the execution reports.

• L1 and L2 providers in the data fetcher were being dropped before the data was being fetched for long-running cost estimator tasks.
• RAM usage would grow to 300-400GB for large cost estimator runs because the intermediate report data was not being dropped. Periodically add the data instead of collecting all of the execution report data at the end to avoid high memory usage.