feat: implement eth_feeHistory endpoint

[ptrus]

Fixes #501

Implements eth_feeHistory endpoint. The query supports returning the history for the maximum last 20 blocks (this arbitrarily set limit is allowed as per spec: Requested range of blocks. Clients will return less than the requested range if not all blocks are available.)

To avoid fetching multiple blocks, transactions, and receipts from DB on every fee_history query (which can be expensive) the fee_history for last 20 blocks is continuously pre-computed in the gas oracle (only the percentiles are computed on the fly).

TODO:

• test

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 44 lines in your changes are missing coverage. Please review.

Project coverage is 62.45%. Comparing base (1c3f6a2) to head (bbe0ba6).
Report is 70 commits behind head on main.

❗ Current head bbe0ba6 differs from pull request most recent head 7c10b2e. Consider uploading reports for the commit 7c10b2e to get more accurate results

Files	Patch %	Lines
gas/backend.go	68.80%	28 Missing and 6 partials ⚠️
rpc/eth/api.go	41.17%	7 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #502      +/-   ##
==========================================
+ Coverage   62.31%   62.45%   +0.13%     
==========================================
  Files          38       37       -1     
  Lines        3962     4051      +89     
==========================================
+ Hits         2469     2530      +61     
- Misses       1285     1307      +22     
- Partials      208      214       +6     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[CedarMist]

Metamask uses eth_feeHistory to calculate the different transaction priorities for gas price.

See metamask source:

• calculateEstimatesForPriorityLevel
• fetchBlockFeeHistory

This doesn't include the current txpool as BTC fee estimation does.

Recently we had a situation where many transactions were queued up, and we were not sure if the fee estimation was returning accurate data, or if the 'Speed Up' button in MetaMask would work correctly.

If eth_feeHistory isn't supported it will fallbcak to eth_gasPrice, in an ideal situation would be to return the average of the minimum and median for having the transaction included within the next block.