[MM-55323] Allow end users to fetch the group members list of groups which allow @-mentions (#26551)

* Allow end users to fetch the group members list of groups which allow @-mentions

* Update server/channels/api4/group_test.go

Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>

* Fix test name

* Move into subtest

---------

Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>

server/public/model/client4.go

@@ -7690,6 +7690,19 @@ func (c *Client4) PatchGroup(ctx context.Context, groupID string, patch *GroupPa
 	return &g, BuildResponse(r), nil
 }
 
+func (c *Client4) GetGroupMembers(ctx context.Context, groupID string) (*GroupMemberList, *Response, error) {
+	r, err := c.DoAPIGet(ctx, c.groupRoute(groupID)+"/members", "")
+	if err != nil {
+		return nil, BuildResponse(r), err
+	}
+	defer closeBody(r)
+	var ml GroupMemberList
+	if err := json.NewDecoder(r.Body).Decode(&ml); err != nil {
+		return nil, nil, NewAppError("UpsertGroupMembers", "api.unmarshal_error", nil, "", http.StatusInternalServerError).Wrap(err)
+	}
+	return &ml, BuildResponse(r), nil
+}
+
 func (c *Client4) UpsertGroupMembers(ctx context.Context, groupID string, userIds *GroupModifyMembers) ([]*GroupMember, *Response, error) {
 	payload, err := json.Marshal(userIds)
 	if err != nil {

server/public/model/group_member.go

@@ -21,3 +21,8 @@ func (gm *GroupMember) IsValid() *AppError {
 	}
 	return nil
 }
+
+type GroupMemberList struct {
+	Members []*User `json:"members"`
+	Count   int     `json:"total_member_count"`
+}