allow modules to set custom stats (#18946)

can be per run or per host, also aggregate or not
set_stats action plugin as reference implementation
added doc stub
display stats in calblack
made custom stats showing configurable

examples/ansible.cfg

@@ -276,6 +276,9 @@
 # it is False, then the last specified argument is used and the others are ignored.
 #merge_multiple_cli_flags = False
 
+# Controls showing custom stats at the end, off by default
+#show_custom_stats = True
+
 [privilege_escalation]
 #become=True
 #become_method=sudo