[Request] Allow users to pass in the output of BaseDaskJobQueueBackend._get_extra_cluster_kwargs

[lilyminium]

This will allow users to customise the initiation of a Dask Jobqueue flexibly. An example use case is the hpc3 cluster at UCI, where you might want to launch a sentinel EvaluatorServer from the cheaper dmobley_lab account but start up dask workers on the necessary yet expensive dmobley_lab_gpu account. Others include e.g. wanting a shebang.