Returns vector of available dates from MRAN or local MRAN repository.
getValidSnapshots(mranRootUrl = mranUrl())MRAN root. This can be a URL, e.g. https://mran.microsoft.com/snapshot/ or the path to a local MRAN repository, e.g.file:///local/path
Character vector with dates of valid snapshots
Other checkpoint functions: checkpointArchives,
  checkpointRemove, checkpoint,
  getAccessDate, mranUrl,
  setSnapshot, unCheckpoint