Add a name to the pandas series returned for feature importance

@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.