Allow to pass string as a shell override

[krassowski]

Relates to jupyterlab/jupyterlab#12540.

I think that for many users passing a sting may be more convenient and for majority (if not all) shlex.split should do the job well.

It will require a follow up PR in jupyter_server to change:

terminado_settings = Dict(List())

(once jupyter-server/jupyter_server#949 gets merged) into

terminado_settings = Dict(Union(List(), Unicode()))

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[blink1073]

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