Adds `certs-set` and `certs-remove` plugin triggers so other plugins can install or remove an app's SSL cert/key pair without shelling out to the `dokku certs:add` / `dokku certs:remove` subcommands. Shared implementations live as `fn-certs-set` and `fn-certs-remove` in `plugins/certs/internal-functions`, with the subcommands and the new triggers calling `verify_app_name` before delegating.
