add product variant by sku accessor

[jenschude]

No description provided.

[codecov]

Codecov Report

Merging #64 (82e29e8) into master (f0d2415) will increase coverage by 0.02%.
The diff coverage is 94.73%.

❗ Current head 82e29e8 differs from pull request most recent head c7acbac. Consider uploading reports for the commit c7acbac to get more accurate results

@@             Coverage Diff              @@
##             master      #64      +/-   ##
============================================
+ Coverage     11.69%   11.71%   +0.02%     
- Complexity    69463    69472       +9     
============================================
  Files          6296     6297       +1     
  Lines        172449   172468      +19     
============================================
+ Hits          20160    20208      +48     
+ Misses       152289   152260      -29     

Impacted Files	Coverage Δ
...c/Api/Models/Product/ProductVariantSkuAccessor.php	94.73% <94.73%> (ø)
...-api/src/Models/Product/ProductProjectionModel.php	14.95% <0.00%> (+2.56%)	⬆️
lib/commercetools-base/src/Base/MapperSequence.php	51.47% <0.00%> (+5.88%)	⬆️
...ols-api/src/Models/Product/ProductVariantModel.php	25.61% <0.00%> (+9.91%)	⬆️
...pi/src/Models/Product/ProductVariantCollection.php	61.53% <0.00%> (+61.53%)	⬆️

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