feat: supply tidyselection for sum_groups_epi_df

[dajmcdon]

Checklist

Please:

• Make sure this PR is against "dev", not "main" (unless this is a release
  PR).
• Request a review from one of the current main reviewers:
  brookslogan, nmdefries.
• Makes sure to bump the version number in DESCRIPTION. Always increment
  the patch version number (the third number), unless you are making a
  release PR from dev to main, in which case increment the minor version
  number (the second number).
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  (backwards-incompatible changes to the documented interface) are noted.
  Collect the changes under the next release number (e.g. if you are on
  1.7.2, then write your changes under the 1.8 heading).

Magic GitHub syntax to mark associated Issue(s) as resolved when this is merged into the default branch

• Resolves Alter sum_groups_epi_df() to take tidyselection instead of chr vctr #655

[brookslogan]

Made some tweaks to the roxygen. @dshemetov , this has the potential to raise warnings or, in very unlikely unfortunate cases, change behavior, in forecast production.

• The changed behavior would be if you did x %>% sum_groups_epi_df(my_vars, .....) and x happened to have a column named my_vars.
• Warnings would be if you did the above but x luckily didn't have a column named my_vars and so it realized it should look at the variable my_vars.
• Resolution is to move to all_of(my_vars) or !!my_vars.

I'll leave it to you to control the merge time in light of the above.