Draft: MetaReader: enable reading some in-file metadata from RNTuple
In the context of experimenting with storage technology for metadata containers, this MR adds some, very limited support for reading metadata payload from RNTuple container.
The main point of it is to allow (at least to some extent) configuring (and running) the jobs by reading some basic metadata information from RNTuple.
Admittedly, the way of retrieving the payload of interest is very hacky and not particularly maintainable (although much faster than currently), however allows for more testing of the metadata in RNTuple.
As a corollary of above points, I don't intend to this to be merged anytime soon, at least in the current shape.