Add `preload` and `preload_argv` arguments to `Client.__init__`

[bryanwweber]

PR #5773 adds the ability for the Client to run preload code, similar to the Worker and Scheduler. However, @jrbourbeau pointed out that users may be passing preload and preload_argv to the Client.__init__ with the intention of passing them to the worker configuration of LocalCluster.

As a temporary measure, #5773 only adds Client preload code that is specified in the Dask YAML configuration file. Since a similar problem with keyword arguments passthrough will happen if/when plugins are added for the Client, @jrbourbeau proposed deferring adding the preload and preload_argv keywords. When those are added, there will need to be a deprecation cycle to change how users pass kwargs to the worker configuration through LocalCluster. This deprecation cycle would also remove the similar problem that will occur when/if plugins are added.

One possible solution to this is to modify the signature of LocalCluster.__init__ so that the **worker_kwargs that are collected at the end of the constructor become a keyword argument of their own, similar to the existing scheduler_kwargs. See

distributed/distributed/deploy/local.py

Lines 121 to 123 in 8d0df89

	scheduler_kwargs=None,
	scheduler_sync_interval=1,
	**worker_kwargs,

This should not affect the n_workers and threads_per_worker kwargs, which would still be passed through unmodified.