healthcheck: add support at the load-balancers of services level

Co-authored-by: Dmitry Sharshakov <d3dx12.xx@gmail.com>
Co-authored-by: Julien Salleyron <julien.salleyron@gmail.com>
Co-authored-by: Jean-Baptiste Doumenjou <925513+jbdoumenjou@users.noreply.github.com>
Co-authored-by: Romain <rtribotte@users.noreply.github.com>
Co-authored-by: Tom Moulard <tom.moulard@traefik.io>

docs/content/reference/dynamic-configuration/file.yaml

@@ -75,6 +75,7 @@ http:
       mirroring:
         service: foobar
         maxBodySize: 42
+        healthCheck: {}
         mirrors:
         - name: foobar
           percent: 42
@@ -82,6 +83,7 @@ http:
           percent: 42
     Service03:
       weighted:
+        healthCheck: {}
         services:
         - name: foobar
           weight: 42