"Watchers" for PRs as well as "Reviewers"

In my organization, I often want to be aware of pull requests for which I'm not necessarily an appropriate reviewer. Having only "Reviewers" as a category makes it hard to signal whether I'm actually expected to pass judgement on a pull request or am have only been included as a courtesy/FYI. It would be nice to also have "Watchers" who would basically just be cc'ed on emails relating to the PR. (This could be implemented as a flag column added to the existing reviewer data structure.) I checked the Marketplace and didn't see anything like this.