Add support for reading/writing POOL internal containers in RNTuple format (and more)

This MR:

• Makes the default container type for the ROOT main technology configurable via PoolSvc (defaults to TTreeIndex), and
• Adds support for reading/writing ## containers in RNTuple format (controlled through the above mentioned configurable).

The type of the ## containers is autodetected for the input files and can be configured for the output files using the appropriate configuration flag, e.g., flags.PoolSvc.DefaultContainerType="ROOTRNTUPLE" writes the ## containers in RNTuple format. Reading/writing ## containers in RNTuple is fully working in main--dev3LCG nightly at the time of this MR.

Closes ATEAM-926

cc: @gemmeren @mnowak