Support @ElementCollection attribute mappings

[igdianov]

Fixes #33

[codecov-io]

Codecov Report

Merging #34 into master will increase coverage by 0.48%.
The diff coverage is 77.77%.

@@             Coverage Diff              @@
##             master      #34      +/-   ##
============================================
+ Coverage     61.73%   62.22%   +0.48%     
- Complexity      212      223      +11     
============================================
  Files            21       21              
  Lines          1427     1432       +5     
  Branches        203      205       +2     
============================================
+ Hits            881      891      +10     
+ Misses          444      442       -2     
+ Partials        102       99       -3

Impacted Files	Coverage Δ	Complexity Δ
...jpa/query/schema/impl/GraphQLJpaSchemaBuilder.java	85.41% <77.77%> (+1.54%)	57 <12> (+11)	⬆️

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