Generalize optimized epi_slide fn

[nmdefries]

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).
• Describe changes made in NEWS.md, making sure breaking changes
  (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).
• See DEVELOPMENT.md for more information on the development
  process.

Change explanations for reviewer

Make epi_slide_mean more general. Let the user pass a data.table rolling fn (frollmean, frollsum, frollapply) or slider rolling function (slide_sum, etc) to expand optimized slide functionality.

[dsweber2]

Other than this bit with slider seems good

[dsweber2]

Looks like the linter and styler disagree about the spacing for this:

  if (inherits(x$time_value, c("yearquarter", "yearweek", "yearmonth")) ||
    is.numeric(x$time_value)) {

so I guess the linter is going to fail indefinitely. Disappointing and weird.

[dsweber2]

Should we wait for Logan to get back? Wouldn't put it past him to have some arcane solution to the identical vs all.equal problems. I'd be fine with leaving that as an open issue for a potential future feature though. Actually using anons is mostly covered by passing args.

[nmdefries]

David opened an issue RE styler here

Will have Logan take a look at this when he gets back, but if it needs changes, I'll do those in a separate PR. I don't want this to be hanging as a PR for a long time.

[nmdefries]

• demonstrate use of new functions in vignettes