hardhat quantiles

[dajmcdon]

Checklist

Please:

• Make sure this PR is against "dev", not "main".
• Request a review from one of the current epipredict main reviewers:
  dajmcdon.
• Make sure to bump the version number in DESCRIPTION and NEWS.md.
  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
  0.7.2, then write your changes under the 0.8 heading).

Change explanations for reviewer

This PR replaces dist_quantiles(), a custom distribution related to the distributional package, with quantile_pred() from the hardhat package. This is one step in off-loading some custom components to core tidymodels functionality.

Replaces #442

[dsweber2]

Generally lgtm, very minor notes and added a test for pivot_longer with more than 1 row.