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

In https://github.com/googleapis/api-linter/pull/1158#discussion_r1207490134, 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.