Implement Modified Event log property

Refer to https://gerrit.openbmc-project.xyz/c/openbmc/phosphor-dbus-interfaces/+/29734


and implement Modified Event log property

Tested:
curl -k -H "X-Auth-Token: $token" https://${bmc}/redfish/v1/Systems/system/LogServices/EventLog/Entries/1
{
  "@odata.id": "/redfish/v1/Systems/system/LogServices/EventLog/Entries/1",
  "@odata.type": "#LogEntry.v1_6_0.LogEntry",
  "Created": "1970-01-01T00:01:35+00:00",
  "EntryType": "Event",
  "Id": "1",
  "Message": "xyz.openbmc_project.Common.Device.Error.ReadFailure",
  "Modified": "1970-01-01T00:01:35+00:00",
  "Name": "System Event Log Entry",
  "Severity": "Critical"
}

Passed the validator:
VERBO - ServiceRoot -> Systems.Systems -> Members.ComputerSystem#0 ->
LogServices.LogServices -> Members.LogService#0 -> Entries.Entries ->
Members.LogEntry#0, LogEntry.v1_6_1, LogEntry
VERBO - @odata.id                                               PASS
VERBO - @odata.type                                             PASS
VERBO - Created                                                 PASS
VERBO - EntryType                                               PASS
VERBO - Id                                                      PASS
VERBO - Message                                                 PASS
VERBO - Modified                                                PASS
VERBO - Name                                                    PASS
VERBO - Severity                                                PASS

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: I5a59a298e95e78acaad11a99558f9046675820d3