feat(GH-204): added LOWER predicate for case insensitive match

[igdianov]

This PR adds LOWER predicate for case insensitive match, i.e. given query

query {
  Books(where: {title: {LOWER: "WAR AND PEACE"}}) {
    select {
      id
      title
    }
  }
}

the expected result is

{
  "data": {
    "Books": {
      "select": [
        {
          "id": 2,
          "title": "War and Peace"
        }
      ]
    }
  }
}

Fixes #204

[codecov]

Codecov Report

Merging #206 into master will increase coverage by 0.11%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #206      +/-   ##
============================================
+ Coverage     73.64%   73.76%   +0.11%     
- Complexity      876      878       +2     
============================================
  Files            50       50              
  Lines          3711     3720       +9     
  Branches        626      627       +1     
============================================
+ Hits           2733     2744      +11     
+ Misses          699      698       -1     
+ Partials        279      278       -1

Impacted Files	Coverage Δ	Complexity Δ
...graphql/jpa/query/schema/impl/PredicateFilter.java	100% <100%> (ø)	5 <0> (ø)	⬇️
...hql/jpa/query/schema/impl/JpaPredicateBuilder.java	58.87% <100%> (+0.27%)	110 <0> (+1)	⬆️
...jpa/query/schema/impl/GraphQLJpaSchemaBuilder.java	91.27% <100%> (+0.07%)	118 <0> (ø)	⬇️
.../graphql/jpa/query/introspection/Constructors.java	82% <0%> (+4%)	15% <0%> (+1%)	⬆️

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