Add support for customizing Testcontainers creation code

[mhalbritter]

We want to generate code like this:

@Bean
@ServiceConnection(type = RabbitStreamConnectionDetails.class)
RabbitMQContainer rabbitContainer() {
return new RabbitMQContainer(DockerImageName.parse("rabbitmq:latest"))
		.withExposedPorts(5552)
		.withCopyToContainer(Transferable.of("[rabbitmq_stream]."), "/etc/rabbitmq/enabled_plugins");
}