Make add_glance_*(galnce_fun) an S3 generic default

[ddsjoberg]

Current the default value is galnce_fun = broom::glance, and we do a little trick inside that if the user didn't specify anything AND it's a mice model, we replace the default with

if (missing(glance_fun) && inherits(x$inputs$x, "mice")) {
    check_pkg_installed("mice", reference_pkg = "gtsummary")
    glance_fun <- \(x) broom::glance(mice::pool(x))
  }

As we plan to decouple many of the gtsummary dependencies, this default should be an S3 generic so it can easily be extended in a supplementary package and gtsummary won't have {mice} listed as a Suggested package.