[FEA] Expose ability to configure cudf's copy on write in LocalCUDACluster

[randerzander]

In internal benchmarks, we're seeing about a 20% runtime improvement when using cudf's copy on write support. Today you can enable it on all workers manually:

def enable_cow():
  import cudf
  cudf.set_option("copy_on_write", True)

client.run(enable_cow)

It'd be nice to do this w/ an arg to LocalCUDACluster, possibly on by default.

cc @galipremsagar , @VibhuJawa , @beckernick