2960/Turn product attributes / variant attributes into a flat list

[Pacu2]

Turned product attributes & variant attributes into a flat list

close #2960

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@fe53197). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #2980   +/-   ##
=========================================
  Coverage          ?   89.72%           
=========================================
  Files             ?      208           
  Lines             ?    10921           
  Branches          ?     1072           
=========================================
  Hits              ?     9799           
  Misses            ?      794           
  Partials          ?      328

Impacted Files	Coverage Δ
saleor/graphql/product/types.py	96.66% <100%> (ø)

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