update the fee estimator proxy to consider Stacks node tx estimated costs

[rafaelcr]

The current fee estimator proxy uses a previous tenure fullness heuristic to figure out transaction fees. However, given recent chain activity, we should adjust it so it also takes the estimated_cost returned by the node into account in order to try and differentiate fee markets between contract calls that may be expensive vs token transfers that are essentially free.

Alternatively, we could consider storing the estimated cost to our mempool tx table in order to try to determine if all the pending txs could be included in the current tenure. This could be tricky, though, because we won't see estimate calls for all mempool txs so we could end up needing to do those calls manually.