Manager: add ManagerDiagnosticData handler

This change adds a link in the Manager for all BMCs to an empty
ManagerDiagnosticData resource and a minimum ManagerDiagnosticData
handler.

This service is backed by phosphor-health-monitor (PHM), which is
enabled by default through the "obmc-apps" package group. If PHM is
disabled, the resource will be empty.

$ curl http://${bmc}:10080/redfish/v1/Managers/bmc/ManagerDiagnosticData
{
  "@odata.id": "/redfish/v1/Managers/bmc/ManagerDiagnosticData",
  "@odata.type": "#ManagerDiagnosticData.v1_0_0.ManagerDiagnosticData",
  "Id": "ManagerDiagnosticData",
  "Name": "Manager Diagnostic Data"
}

Also ran the Redfish Service Validator to make sure no new errors are
introduced with the introduction of ManagerDiagnosticData.

Signed-off-by: Sui Chen <suichen@google.com>
Change-Id: Iba242bc3b6ebec851dbd26e149d5c92c19a7992e