add metadata to entry in replica catalog

In the replica catalog, we will need to add metadata in there. As it stands, a replica catalog entry is as follows:
{
  site: <string>,
  lfn: <string>,
  pfn: <string>,
  checksum: {
    type: <string>,
    value: <string>
  }
}

Per 5/18 dev meeting (https://docs.google.com/document/d/1URPG6SLG481Y7DDAajjVtedKjS5pwIQ41UOhwSQYbhk/edit#bookmark=id.6b8rt2t6hoti) we have decided to go with:

replicas:
[
{
  lfn: <string>,
  pfns: [
    {
      site: <string>,
      path: <string>
    }, ...
  ],
  checksum: {
    sha256: <string>
  },
  metadata: {
    k: v,
   ...
  }
}
]