better access to the fee estimation

[vincenzopalazzo]

Currently, if we want to estimate a fee in Nakamoto, we need to require a block, and Nakamoto keeps track of the fee with an estimation logic. In this PR (#139), I added a naive fee estimation, but some work needs to be done, such as:

• Not storing all the estimated fees for all blocks in memory: after a certain limit, we should cut the hash map.
• Being able to persist the calculated fee: We should be able to store the calculated fee to enable quick responses across restarts. See nakamoto can not estimate fee after restarts coffee-tools/folgore#20 for a use case.

[vincenzopalazzo]

While I was talking about estimation fees in nakamoto Inoted that Neutrino used the BIP133 to get the minimal fee, so we should explore this solution too