Support multi-host in PostCodes Redfish LogServices.

- In handleLogServices, register PostCodes for multi-host systems using
  parsePostCodeHostIndex and dynamic URLs.
- Update handleGetPostCodesLogService, handlePostPostCodesClear,
  fillPostCodeEntry, getPostCodeForEntry, getPostCodeForBoot,
  getCurrentBootNumber, handleGetPostCodesEntryCollection,
  handleGetPostCodesEntryAdditionalData, and handleGetPostCodesEntry
  to resolve system information via system_utils::getSystemInformation
  and dynamically target the corresponding D-Bus PostCode service
  and object path.
- Declare handleGetPostCodesLogService, handlePostPostCodesClear,
  handleGetPostCodesEntryCollection, handleGetPostCodesEntryAdditionalData,
  and handleGetPostCodesEntry in log_services.h without inline to allow
  external linkage and direct unit testing in log_services_test.cc.

Google-Bug-Id:543170788
PiperOrigin-RevId: 962563019
Change-Id: I59af81c43e463207ba6f9fe071615dcc2efdc8e8