REST API: Include sudo-ability in GET response on /v2/users & create endpoint to alter sudo-ability of existing user

Currently the snapd REST API has a means of extracting information about user accounts (https://snapcraft.io/docs/snapd-api#heading--users), but this information doesn't include the users' sudo-ability. Since the API allows creating users with a given sudo-ability, it would be nice to have this information included in the GET response.

If changing the sudo-ability one currently needs to remove the user then re-create with the updated sudo-ability state. Adding a new endpoint or "modify" action to the POST of "/v2/users" would simplify the action.