feat: add support for variable where criteria expressions

[igdianov]

This PR adds support for variable where criteria expressions, i.e. given

query($where: BooksCriteriaExpression) {
  Books (where: $where) {
    select {
      id(orderBy: ASC)
      title
      genre
    }
  }
}

with variables payload:

{
  "where": { 
    "genre": {
      "IN": ["NOVEL"]
    }
  }
}

the expected result will be:

{
  "data": {
    "Books": {
      "select": [
        {
          "id": 2,
          "title": "War and Peace",
          "genre": "NOVEL"
        },
        {
          "id": 3,
          "title": "Anna Karenina",
          "genre": "NOVEL"
        }
      ]
    }
  }
}

Fixes #126

[igdianov]

@anotender Here is the first crack of the support passing where criteria expressions via variables. Feel free to try adding any test cases for your requirements...

[codecov]

Codecov Report

Merging #162 into master will increase coverage by 0.52%.
The diff coverage is 83.33%.

@@             Coverage Diff              @@
##             master     #162      +/-   ##
============================================
+ Coverage     68.12%   68.65%   +0.52%     
- Complexity      502      510       +8     
============================================
  Files            33       33              
  Lines          2576     2597      +21     
  Branches        431      431              
============================================
+ Hits           1755     1783      +28     
+ Misses          644      643       -1     
+ Partials        177      171       -6

Impacted Files	Coverage Δ	Complexity Δ
...a/query/schema/impl/QraphQLJpaBaseDataFetcher.java	74.95% <83.33%> (+0.97%)	160 <10> (+4)	⬆️
...hql/jpa/query/schema/impl/JpaPredicateBuilder.java	55.86% <0%> (+1.4%)	108% <0%> (+3%)	⬆️
...graphql/jpa/query/schema/impl/PredicateFilter.java	100% <0%> (+3.03%)	5% <0%> (+1%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8028d12...dd53c6e. Read the comment docs.

[anotender]

@igdianov as for me, the changes look ok at the moment. Do you want to add something more?
And one more thing, what formatter should I use? As the default Intellij one seems to not format files as it should.

[igdianov]

@anotender Thanks! Everything looks good. I will merge and release this on the weekend. I am using STS 4.0 with custom formatter. I don't know how to export to IDEA. I will try to find a matching IDEA configuration.