User: add interfaces and errors for groups

As proposed in the following design,
[1] https://github.com/openbmc/docs/blob/master/designs/redfish-authorization.md

The UserManager interface needs to expose new interfaces to add/delete
secondary groups, which are then used to model Redfish roles and
privileges.

An implementation is in the follow code review,
[1] https://gerrit.openbmc.org/c/openbmc/phosphor-user-manager/+/58143.

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: I252e56dc03e694e3aedf3ae6fdda64edc947fc06
diff --git a/yaml/xyz/openbmc_project/User/Common.metadata.yaml b/yaml/xyz/openbmc_project/User/Common.metadata.yaml
index 1d7a58f..5e20ac1 100644
--- a/yaml/xyz/openbmc_project/User/Common.metadata.yaml
+++ b/yaml/xyz/openbmc_project/User/Common.metadata.yaml
@@ -17,3 +17,7 @@
   meta:
       - str: "REASON=%s"
         type: string
+- name: GroupNameExists
+  level: ERR
+- name: GroupNameDoesNotExist
+  level: ERR