phosphor-regulators: Implement presence service

[smccarney]

Design and implement the presence service. This will be used by regulator classes to determine whether FRUs are present in the system.

The real implementation will obtain the presence information from D-Bus. The mock implementation will allow the presence to be specified in the test case to exercise all code paths.

[smccarney]

Design

• Class hierarchy
  • PresenceService
  • DBusPresenceService
  • MockPresenceService
• Methods:
  • bool isPresent(const std::string& inventoryPath)
  • void clearCache()
• Add methods and data members to Services hierarchy
  • Services
  • void clearCache()
  • PresenceService& getPresenceService()
  • BMCServices
  • void clearCache()
  • DBusPresenceService presence;
  • MockServices
  • void clearCache()
  • MockPresenceService& getMockPresenceService()
  • MockPresenceService presence;
• Example Usage:
  • bool isPresent = services.getPresenceService().isPresent(fru);

[smccarney]

Resolved by https://gerrit.openbmc-project.xyz/c/openbmc/phosphor-power/+/37992