feat: allow setting tx mining service apikey

[luislhl]

Acceptance Criteria

• It should be possible to set an api-key in the config to be used for tx-mining-service

Security Checklist

• Make sure you do not include new dependencies in the project unless strictly necessary and do not include dev-dependencies as production ones. More dependencies increase the possibility of one of them being hijacked and affecting us.

[codecov]

Codecov Report

Merging #175 (5fdfe8f) into dev (7a29078) will increase coverage by 0.06%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##              dev     #175      +/-   ##
==========================================
+ Coverage   85.78%   85.84%   +0.06%     
==========================================
  Files          19       19              
  Lines         647      650       +3     
  Branches      132      133       +1     
==========================================
+ Hits          555      558       +3     
  Misses         83       83              
  Partials        9        9              

Impacted Files	Coverage Δ
src/controllers/index.controller.js	72.50% <0.00%> (ø)
src/index.js	71.42% <100.00%> (+4.76%)	⬆️

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[pedroferreira1]

The code for me is good, just two considerations:

1. I think we should wait for @tuliomir's refactor PR (Refactor of ExpressJS files #174). It changed some stuff in the src.index.js file and may change a bit this PR as well.
2. We need to wait for the lib bump before merging

[luislhl]

I'm ignoring Codecov report this time because they are complaining about some console.log that I have replaced with console.error. This can be seen in their report link.