Create a srvyr results data.frame which is automatically unpacked by srvyr
Source:R/survey_statistics_helpers.R
as_srvyr_result_df.Rdsrvyr uses data.frame columns wrapped by this function to know when to
automatically unpack the results for the user. When developing extensions
(see vignette extending-srvyr), use this function to wrap the result
in so that summarize knows to unpack them.