feat: Provide default GraphQLObjectCoercing fallback for non-existing Java classes

[igdianov]

This PR adds support for generating Java Scalar type with default GraphQLObjectCoercing fallback for non-registered Java classes in JavaScalars type registry, i.e.

// given
Class<?> clazz = VariableValue.class;

// when
GraphQLScalarType scalarType = JavaScalars.of(clazz);

// then
assertThat(scalarType.getCoercing()).isInstanceOf(GraphQLObjectCoercing.class);
assertThat(scalarType.getName()).isEqualTo("VariableValue");

It is possible to override default by registering custom Java scalar type as follows:

//given
JavaScalars.register(Map.class, new GraphQLScalarType("Map", "Map Object Type", new GraphQLObjectCoercing()));

//when
GraphQLScalarType scalarType = JavaScalars.of(Map.class);
        
//then
Coercing<?,?> coercing = scalarType.getCoercing();

assertThat(coercing).isInstanceOf(GraphQLObjectCoercing.class);
assertThat(scalarType.getName()).isEqualTo("Map");

[codecov]

Codecov Report

Merging #130 into master will decrease coverage by 0.09%.
The diff coverage is 25%.

@@             Coverage Diff             @@
##             master     #130     +/-   ##
===========================================
- Coverage     68.18%   68.08%   -0.1%     
  Complexity      400      400             
===========================================
  Files            33       33             
  Lines          2065     2068      +3     
  Branches        307      307             
===========================================
  Hits           1408     1408             
- Misses          532      535      +3     
  Partials        125      125

Impacted Files	Coverage Δ	Complexity Δ
...ventures/graphql/jpa/query/schema/JavaScalars.java	45.41% <25%> (-0.58%)	4 <1> (ø)

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