Add `GraphQlClient` and `GraphQlTester` Kotlin extensions

[bclozel]

Both GraphQlClient and GraphQlTester APIs use the ParameterizedType as an argument because of Java's type erasure.
We can contribute Kotlin extensions for such cases to get reified type parameters.

For example this Java code:

Project project = graphQlClient.document(document) 
	.retrieveSync("project") 
	.toEntity(Project.class); 

can become with Kotlin:

val project = graphQlClient.document(document).retrieveSync("project").toEntity<Project>()