AIP-158: should response-plural-first-field only apply to resources?

[noahdietz]

In #1158 (comment), it was pointed out that simply checking if the field in question is plural or not is not a robust check when the field represents a resource. However, AIP-158 isn't exclusively for resources, so perhaps it shouldn't only focus on them. So while we can handle the case where the field represents a resource and check its plural, maybe it shouldn't be limited to that. Not sure though, we want to chat about it some more.