Improve getLabel functions

[seancolsen]

This PR cleans up getLabel props for a few components, with the following goals.

• Don't use a labelKey argument within the getLabel prop.

  Rationale: The consuming component has two ways of customizing the label. Either (a) via the labelKey prop, or (b) via the getLabel prop. When using (b), then (a) should be irrelevant. The getLabel function signature is simpler when it does not concern itself with labelKey.

• Allow getLabel to return an instance of ComponentAndProps.

• Make all getLabel props consistently use the type LabelGetter<Option> across all relevant components.

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[pavish]

Looks good!

[seancolsen]

Thanks for the review @pavish. Kriti and I are currently trying to figure out how to get this merged without the readthedocs check passing.