Implement post code manager

This depends on interfaces definition:
https://gerrit.openbmc-project.xyz/c/openbmc/phosphor-dbus-interfaces/+/20474

Implement method and properties defined in PostCode.interface.yaml
under phosphor-dbus-interfaces/xyz/openbmc_project/State/Boot
1. Method: std::vector<uint64_t> PostCode::getPostCodes(uint16_t index)
2. Properties: CurrentBootCycleIndex/MaxBootCycleNum

Test-By:
    Every cycle post codes is saved in "/var/lib/phosphor-post-code-manager"
    "1" file is saved all post codes for cycle 1
    "2" file is saved all post codes for cycle 2
    "CurrentBootCycleIndex" file is saved the current boot cycle number.
    root@wolfpass:/var/lib/phosphor-post-code-manager# ls
      1  2 CurrentBootCycleIndex

Change-Id: Ia89b9121983261fef5573092d890beb84626ceeb
Signed-off-by: Kuiying Wang <kuiying.wang@intel.com>
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new file mode 100644
index 0000000..dd27708
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