[tune] add output flag for Tune CLI

[andrewztan]

What do these changes do?

Adds output flag (-o / --output) to ls / lsx commands for Tune CLI. Builds on top of #3983

[andrewztan]

@richardliaw

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[hartikainen]

Would it be better to output these in a csv instead of pkl? Considering that the data being outputted does not need to have python objects saved, i.e. the data is tabular, I think it would be better serialize the things with something else than pickle.

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Can one of the admins verify this patch?

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Can one of the admins verify this patch?

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[andrewztan]

@richardliaw @hartikainen ready for review / merge

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[hartikainen]

Looks good!

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[richardliaw]

We should add this to the docs.