make 'summarize_dt' work with multiple value columns

Describe changes

Make summarize_dt work with multiple value columns so that the user doesn't have to manually call the function multiple times.

This has a breaking change, since the argument value_col is changed to value_cols. Here are all the places that would need to be updated https://github.com/search?p=2&q=org%3Aihmeuw-demographics+summarize_dt&type=Code. It looks like popMethods and anything that used model_template.

Maybe can introduce this change in the next docker builds?

[X] Have you read the contributing guidelines for ihmeuw-demographics R packages?
[X] Have you successfully run devtools::check() locally?
[X] Have you updated or added function (and vignette if applicable) documentation? Did you update the 'man' and 'NAMESPACE' files with devtools::document()?
[X] Have you added in tests for the changes included in the PR?
[X] Do the changes follow the ihmeuw-demographics code style?
[ ] Do the changes need to be immediately included in a new build of docker-base or docker-internal? If so follow directions in those repositories to rebuild and redeploy the images.
[X] Do the changes require updates to other repositories which use this package? If yes, make the necessary updates in those repos, and consider integration tests for those repositories.