Add a name to the pandas series returned for feature importance

[angela97lin]

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@ParthivNaresh asked in #3305: Do we want to add a name to the feature importance series?

Originally posted by @ParthivNaresh in https://github.com/alteryx/evalml/pull/3305#r805906013

I don't think it's a bad suggestion! We could update the name of the series returned by feature importance to have a name field called "feature_importance" or something of the like.