Add SchemaFinder to enable directory support in maven/gradle plugins

[joffrey-bion]

This didn't require any change in the current GraphqlCodegen implementation after all.
We can actually just use the SchemaFinder from the caller code (plugin) to get a list of schemas, and then call the generator with that list.

Resolves:
#30

[codecov]

Codecov Report

Merging #32 into master will increase coverage by 0.52%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master      #32      +/-   ##
============================================
+ Coverage     87.33%   87.86%   +0.52%     
- Complexity      220      231      +11     
============================================
  Files            29       30       +1     
  Lines           529      552      +23     
  Branches         74       76       +2     
============================================
+ Hits            462      485      +23     
  Misses           44       44              
  Partials         23       23              

Impacted Files	Coverage Δ	Complexity Δ
...lynskyi/graphql/codegen/supplier/SchemaFinder.java	100.00% <100.00%> (ø)	11.00 <11.00> (?)

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[kobylynskyi]

@joffrey-bion this looks great! 🏆
BTW, I see you are still making some changes to the tests, so please let me know when I can merge this.

[joffrey-bion]

I'm done with the tests, there was an issue about file traversal order. On the CI the order was not the same.
I realized that the depth-first order of the spec didn't guarantee anything about same-level order.

I fixed the tests by sorting the returned schemas alphabetically

[joffrey-bion]

Thanks a lot for merging! I'll make changes to the plugins once a new version of the core lib is released

[kobylynskyi]

I will try to work on #33 first if you don't mind. So that you can make changes in the same repo.

[joffrey-bion]

@kobylynskyi sure that's totally fine. It's really nice that you're doing this. Thanks a lot for your reactivity and availability.