[ntuple, io] pass paths, names of ntuples for merging to RNTuple side

[mxxo]

We construct a vector of string pairs and pass it through to the
RNTuple::Merge function for merging. This implements a "legacy bridge"
between hadd/TFileMerger and the new RNTuple library.

We adhere to the Merge(TCollection*, TFileMergeInfo*) interface without
using either class in the implementation by carefully casting the
RNTuple information vector and output file string pointers.

An example of this technique is shown here, which passes UBSan, ASan,
and strict aliasing checks on gcc 10 and clang 10: https://godbolt.org/z/7ff3e8

For demonstration, the hadd output for merging two ntuple files looks like:

$ ./bin/hadd -f dir/merged.root ntuple1.root ntuple2.root
hadd Target file: dir/merged.root
hadd compression setting for all output: 1
hadd Source file 1: ntuple1.root
hadd Source file 2: ntuple2.root
hadd Sources and Target have different compression levels
hadd merging will be slower
hadd Target path: dir/merged.root:/
Warning in <TFileMerger::MergeRecursive>: merging RNTuples is experimental

# note: TFileMerger debug output begins 
examining ntuple1.root:/
        got key for type ROOT::Experimental::RNTuple with name ntuple
examining ntuple2.root:/
        got key for type ROOT::Experimental::RNTuple with name ntuple
got ntuple from file 'ntuple1.root:/' named 'ntuple'
got ntuple from file 'ntuple2.root:/' named 'ntuple'

# note: RNTuple::Merge debug output begins
RNTuple merger output file is dir/merged.root:/
merging ntuple from file 'ntuple1.root:/ named 'ntuple'
merging ntuple from file 'ntuple2.root:/ named 'ntuple'
RNTuple merging is unimplemented

# control is passed back to TFileMerger::MergeRecursive
Error in <TFileMerger::MergeRecursive>: error merging RNTuples
Error in <TFileMerger::Merge>: error during merge of your ROOT files

[phsft-bot]

Starting build on ROOT-debian10-i386/cxx14, ROOT-performance-centos7-multicore/default, ROOT-fedora30/cxx14, ROOT-fedora31/noimt, ROOT-ubuntu16/nortcxxmod, mac1014/python3, mac1015/cxx17, windows10/cxx14
How to customize builds

[jblomer]

I think we should leave that open for information but open a separate PR with the merging parts that are RNTuple-internal. We can then merge the RNTuple-internal parts independently but still use the code in this PR to (manually) test if the merging still works as if called from hadd.