Create an example metadata handler

[ponyisi]

This may possibly fit better in a contrib package.

Once we have per-analysis download directories with their own cache database files (#498) then we should have an example "dataset handler" that is able to expose the cache DB information easily to other packages. For example, it can set up links with human-readable names to the various sample directories, clean up outdated samples, maintain multiple versions, and export information in other formats e.g. as parsed by RooDataFrame (root-project/root#11624).

We want to make this pluggable, so it needs to use documented interfaces to the DB information.