add mas healthcheck

mas.config.yaml.tmpl

@@ -19,6 +19,8 @@ http:
         - name: graphql
           playground: false
         - name: assets
+        # https://element-hq.github.io/matrix-authentication-service/reference/configuration.html#httplisteners
+        - name: health
       binds:
         - address: "[::]:8080"