Fix handling of default values in schema for complex types.

[srinivasankavitha]

The generated code for a schema that sets default values for complex types does not compile. Example test case:

@Test
    fun generateInputWithDefaultValueForComplexType() {
        val schema = """
            enum Color {
                red
            }
            
            input ColorFilter {
                color: Color = red
            }
            
            input Car {
                color: Color = red
                make: String
            }
            
            input MyCar {
                car: Car = {color: red}
            }
        """.trimIndent()

        val (dataTypes, _, enumTypes) = CodeGen(CodeGenConfig(schemas = setOf(schema), packageName = basePackageName)).generate()

        assertCompilesJava(enumTypes + dataTypes)
    }

The code generation was not handling input defaults with type ObjectValue.

This PR fixes this by handling ObjectValue case by instantiating the equivalent class using default constructor.