[0.2] Add badger store for Extension

[flaviodsr]

Implement ExtensionStore interface using Badger as storage.

Backports: #102

[codecov]

Codecov Report

Merging #112 (d04c506) into release-0.2 (4fb4a0d) will decrease coverage by 0.55%.
The diff coverage is 65.47%.

@@               Coverage Diff               @@
##           release-0.2     #112      +/-   ##
===============================================
- Coverage        68.93%   68.38%   -0.56%     
===============================================
  Files                8        9       +1     
  Lines             1323     1575     +252     
===============================================
+ Hits               912     1077     +165     
- Misses             321      379      +58     
- Partials            90      119      +29     

Flag	Coverage Δ
unittests	68.38% <65.47%> (-0.56%)	⬇️

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Impacted Files	Coverage Δ
pkg/core/store/badger/extension.go	65.47% <65.47%> (ø)

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