Add GraphQlTester Authorization Header Support

[rwinch]

It would be nice if GraphQlTester could support authenticating via the Authorization Header. A good place this could be validated is in https://github.com/spring-projects/spring-graphql/blob/cedb5fb40d4b3499efc37019ec7e60d89fdbb9cb/samples/webflux-security/src/test/java/io/spring/sample/graphql/SampleApplicationTests.java