Add frosting to CI and sync with main

[dshemetov]

This is a test, but should really be all the changes we need for #47 .

[dshemetov]

@brookslogan CI is getting this error (got it locally too). Any ideas?

Error: Error: processing vignette 'knn-forecasts.Rmd' failed with diagnostics:
C stack usage  15945684 is too close to the limit

[dshemetov]

Meged main in and there were no conflicts. It looks like @dajmcdon may have already done this in a68e901. This should help with the diffs in other PRs that may have branched off of main instead of frosting (like #48).

• Still need to fix whatever CI stack error is happening in the KNN vignette.

[brookslogan]

Looks good to me, aside from the cleanliness of the as_tibble.epi_df approach which will be addressed in a separate issue. [The original approach was just minimally edited to avoid infinite recursion in the knn vignette when epiprocess and epipredict are both loaded. It is still probably asking for some unintended behavior down the line.]