PEL: Add repository to save PELs

Create the Repository class that can save PELs in (and later retrieve
them from) the filesystem.  It provides an add() method that can add
a PEL object to the repository.

Now, when the Manager class sees an OpenBMC event log created with the
RAWPEL metadata in the AdditionalData property that points at a file
that contains a PEL, it can save that PEL.  Before the PEL is saved, the
log ID and commit timestamp fields in the PEL will be updated - the log
ID to a unique value, and the timestamp to the current time.

Change-Id: I8dbaddf0f155bcb6d40b933294ada83feb75ce53
diff --git a/extensions/openpower-pels/paths.hpp b/extensions/openpower-pels/paths.hpp
index 334165c..64566d2 100644
--- a/extensions/openpower-pels/paths.hpp
+++ b/extensions/openpower-pels/paths.hpp
@@ -11,5 +11,10 @@
  */
 std::filesystem::path getPELIDFile();
 
+/**
+ * @brief Returns the path to the PEL repository
+ */
+std::filesystem::path getPELRepoPath();
+
 } // namespace pels
 } // namespace openpower