feat: fees/priority endpoint

[freemanzMrojo]

Description

This endpoint is a version of eth_maxPriorityFeePerGas.

Interface-wise is the same, and the parameters are based on go-ethereum except for the max priority fee cap (500 gwei in Eth, we do not have a reference figure yet). The other difference is that all the params are internal and non-configurable (in geth they use also flags, we can do the same but I prefer to comment it over the PR).

Re-uses the cache logic of the other fees endpoint so the common cache is built by any of these endpoints.

Added a new integration test for this endpoint, further testing will be done as part of the follow-up ticket regarding parameters. Checked though that the priority fees array is built as expected.

Closes https://github.com/vechain/protocol-board-repo/issues/390

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.55556% with 22 lines in your changes missing coverage. Please review.

Project coverage is 62.16%. Comparing base (1b887cd) to head (ed463d5).

Files with missing lines	Patch %	Lines
api/fees/data.go	71.64%	15 Missing and 4 partials ⚠️
api/fees/fees.go	86.95%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@                  Coverage Diff                  @@
##           release/galactica     #984      +/-   ##
=====================================================
+ Coverage              62.14%   62.16%   +0.02%     
=====================================================
  Files                    226      226              
  Lines                  23498    23570      +72     
=====================================================
+ Hits                   14603    14653      +50     
- Misses                  7761     7779      +18     
- Partials                1134     1138       +4     

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