Add sbt plugin integration test

[benmccann]

The test is failing, but I figured it's worth committing anyway to add the infrastructure. I'll probably need some help from other sbt devs to figure out what's wrong and having it checked in will help with that. I also fixed two issues that it found

One thing that it'd be good to get some review on - are the options that I changed to be of type Option[String] all optional? (i.e. are they all nullable in the Maven/Gradle plugins?)

[codecov]

Codecov Report

Merging #127 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #127      +/-   ##
============================================
- Coverage     90.86%   90.83%   -0.03%     
- Complexity      401      404       +3     
============================================
  Files            49       49              
  Lines           974      993      +19     
  Branches        127      130       +3     
============================================
+ Hits            885      902      +17     
- Misses           58       60       +2     
  Partials         31       31              

Impacted Files	Coverage Δ	Complexity Δ
...obylynskyi/graphql/codegen/mapper/MapperUtils.java	86.48% <0.00%> (-3.17%)	20.00% <0.00%> (+1.00%)	⬇️
...om/kobylynskyi/graphql/codegen/GraphQLCodegen.java	96.92% <0.00%> (+0.02%)	44.00% <0.00%> (ø%)
...per/FieldDefinitionsToResolverDataModelMapper.java	97.87% <0.00%> (+0.19%)	11.00% <0.00%> (+1.00%)
...a/com/kobylynskyi/graphql/codegen/utils/Utils.java	80.48% <0.00%> (+0.48%)	20.00% <0.00%> (ø%)
...apper/FieldDefinitionToRequestDataModelMapper.java	95.23% <0.00%> (+1.48%)	4.00% <0.00%> (+1.00%)

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[benmccann]

The integration test is now passing and the plugin should be ready for prime time!