feat: add average fee to block response

[He1DAr]

Adding a new attribute average_fee to block response, closes #1209

[github-actions[bot]]

🚀 Deployed on https://deploy-preview-1214--stacks-blockchain-api.netlify.app

[rafaelcr]

@He1DAr there still seem to be some issues with lint and some broken tests, let me know if I can help you with anything here

[codecov-commenter]

Codecov Report

Merging #1214 (6e836d4) into develop (8ee1da8) will decrease coverage by 68.67%. The diff coverage is n/a.

:exclamation: Current head 6e836d4 differs from pull request most recent head fb6a149. Consider uploading reports for the commit fb6a149 to get more accurate results

@@             Coverage Diff             @@
##           develop   #1214       +/-   ##
===========================================
- Coverage    76.61%   7.94%   -68.68%     
===========================================
  Files          104     103        -1     
  Lines         9725    9331      -394     
  Branches      2092    1961      -131     
===========================================
- Hits          7451     741     -6710     
- Misses        2164    7941     +5777     
- Partials       110     649      +539     

Impacted Files	Coverage Δ
src/bns-constants.ts	0.00% <0.00%> (-100.00%)	:arrow_down:
src/api/routes/info.ts	0.00% <0.00%> (-100.00%)	:arrow_down:
src/api/routes/fee-rate.ts	0.00% <0.00%> (-100.00%)	:arrow_down:
src/api/routes/stx-supply.ts	0.00% <0.00%> (-100.00%)	:arrow_down:
src/stacks-encoding-helpers.ts	0.00% <0.00%> (-100.00%)	:arrow_down:
src/api/routes/bns/addresses.ts	0.00% <0.00%> (-100.00%)	:arrow_down:
src/api/routes/bns/namespaces.ts	0.00% <0.00%> (-100.00%)	:arrow_down:
src/migrations/1584619633448_txs.ts	0.00% <0.00%> (-100.00%)	:arrow_down:
src/datastore/offline-dummy-store.ts	0.00% <0.00%> (-100.00%)	:arrow_down:
src/migrations/1608030374842_names.ts	0.00% <0.00%> (-100.00%)	:arrow_down:
... and 82 more

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