Allow user to map an attribute to generate a <feature><featurecaption> value for features

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This issue arises to support accessible names for features, to be displayed on hover / focus.

@Malvoz raised this issue originally. Hopefully the <title> <featurecapton> element works to resolve his issue?

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We went with the <featurecaption>, but the UI to enable this in GeoServer is yet TBD.

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This is solved by this commit