feat(GH-235): added Relay Connection support

[igdianov]

This adds experimental support for GraphQL Relay Connection wrapper. The Relay can be enabled with enableRelay(true) method on GraphQLJpaSchemaBuilder instance, i.e.

    @Configuration
    public static class GraphQLJpaQuerySchemaConfigurer implements GraphQLSchemaConfigurer {

        private final EntityManager entityManager;

        @Autowired
        private GraphQLJpaQueryProperties properties;

        public GraphQLJpaQuerySchemaConfigurer(EntityManager entityManager) {
            this.entityManager = entityManager;
        }

        @Override
        public void configure(GraphQLShemaRegistration registry) {
            registry.register(
                    new GraphQLJpaSchemaBuilder(entityManager)
                        .name(properties.getName())
                        .description(properties.getDescription())
                        .useDistinctParameter(properties.isUseDistinctParameter())
                        .defaultDistinct(properties.isDefaultDistinct())
                        .enableRelay(properties.isEnableRelay())
                        .build()
            );
        }
    }    

With Relay enabled, it will generate Relay Connection wrappers for plural entity fileds, i.e.

After that, it will allow to execute queries using Relay schema with opaque cursors:

query {
  books(
    first: 2
    where: {title: {IS_NULL: false}}
  ) {
    pageInfo {
      hasNextPage
      hasPreviousPage
      startCursor
      endCursor
    }
    edges {
      node {
        id
        title
        genre
        tags
        author {
          id
          name
        }
      }
      cursor
    }
  }
}

{
  "data": {
    "books": {
      "pageInfo": {
        "hasNextPage": true,
        "hasPreviousPage": false,
        "startCursor": "b2Zmc2V0PTE=",
        "endCursor": "b2Zmc2V0PTI="
      },
      "edges": [
        {
          "node": {
            "id": 2,
            "title": "War and Peace",
            "genre": "NOVEL",
            "tags": [
              "piece",
              "war"
            ],
            "author": {
              "id": 1,
              "name": "Leo Tolstoy"
            }
          },
          "cursor": "b2Zmc2V0PTE="
        },
        {
          "node": {
            "id": 3,
            "title": "Anna Karenina",
            "genre": "NOVEL",
            "tags": [
              "karenina",
              "anna"
            ],
            "author": {
              "id": 1,
              "name": "Leo Tolstoy"
            }
          },
          "cursor": "b2Zmc2V0PTI="
        }
      ]
    }
  }
}

Fixes #235

[codecov]

Codecov Report

Merging #236 into master will decrease coverage by 1.74%.
The diff coverage is 62.14%.

@@             Coverage Diff              @@
##             master     #236      +/-   ##
============================================
- Coverage     72.41%   70.67%   -1.75%     
- Complexity      919      934      +15     
============================================
  Files            54       62       +8     
  Lines          4180     4351     +171     
  Branches        644      661      +17     
============================================
+ Hits           3027     3075      +48     
- Misses          873      998     +125     
+ Partials        280      278       -2

Impacted Files	Coverage Δ	Complexity Δ
...phql/jpa/query/schema/relay/OffsetBasedCursor.java	0% <0%> (ø)	0 <0> (?)
...es/graphql/jpa/query/schema/relay/GenericPage.java	0% <0%> (ø)	0 <0> (?)
...query/schema/relay/GraphQLJpaRelayDataFetcher.java	0% <0%> (ø)	0 <0> (?)
...ures/graphql/jpa/query/schema/relay/SortField.java	0% <0%> (ø)	0 <0> (?)
...es/graphql/jpa/query/schema/relay/PageFactory.java	0% <0%> (ø)	0 <0> (?)
...raphql/jpa/query/schema/relay/PagingArguments.java	0% <0%> (ø)	0 <0> (?)
...query/schema/impl/GraphQLJpaStreamDataFetcher.java	100% <100%> (+19.51%)	4 <0> (ø)	⬇️
...es/graphql/jpa/query/schema/impl/PageArgument.java	100% <100%> (ø)	4 <4> (?)
.../query/schema/impl/GraphQLJpaQueryDataFetcher.java	91.66% <100%> (+1.14%)	7 <4> (-12)	⬇️
...query/schema/impl/GraphQLJpaSimpleDataFetcher.java	92.3% <100%> (+1.13%)	5 <0> (-6)	⬇️
... and 17 more

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