[ntuple] Schema evolution of `std::variant`

[hahnjo]

Description

We currently have no RVariantField::BeforeConnectPageSource that either checks or correctly schema evolves a std::variant field.

Reproducer

#include <ROOT/RNTupleModel.hxx>
#include <ROOT/RNTupleReader.hxx>
#include <ROOT/RNTupleWriter.hxx>

#include <cstdint>
#include <string>
#include <variant>

void ntuple_variant() {
  {
    auto model = ROOT::RNTupleModel::Create();
    using VarType = std::variant<std::int32_t, std::string>;
    auto var = model->MakeField<VarType>("var");
    auto writer = ROOT::RNTupleWriter::Recreate(std::move(model), "ntpl", "ntuple_variant.root");

    *var = 42;
    writer->Fill();
    *var = "abc";
    writer->Fill();
  }

  { // 1
    auto model = ROOT::RNTupleModel::Create();
    using VarType = std::variant<std::int32_t, std::string, float>;
    auto var = model->MakeField<VarType>("var");
    try {
      auto reader = ROOT::RNTupleReader::Open(std::move(model), "ntpl", "ntuple_variant.root");
    } catch (const ROOT::RException &e) {
      std::cout << e.GetError().GetReport() << "\n";
    }
  }

  { // 2
    auto model = ROOT::RNTupleModel::Create();
    using VarType = std::variant<std::int32_t>;
    auto var = model->MakeField<VarType>("var");
    auto reader = ROOT::RNTupleReader::Open(std::move(model), "ntpl", "ntuple_variant.root");
    reader->LoadEntry(0);
    std::cout << "0: index = " << var->index() << "\n";
    reader->LoadEntry(1);
    std::cout << "1: index = " << var->index() << "\n";
  }
}

This macro writes a std::variant<std::int32_t, std::string> and then demonstrates two problems:

1. When trying to read back as a std::variant<std::int32_t, std::string, float>, the additional float leaf field complains. I think it would be good to automatically support this case.

No on-disk field information for `var._2`
At:
  const ROOT::RFieldBase::ColumnRepresentation_t &ROOT::RFieldBase::EnsureCompatibleColumnTypes(const ROOT::RNTupleDescriptor &, std::uint16_t) const

2. The second case is more severe: Here the user tries to read back into a std::variant<std::int32_t> and this crashes once the on-disk switch tag indicates a value in the second item field:
   

   root/tree/ntuple/src/RFieldMeta.cxx

   Lines 1258 to 1262 in 70c900f

   	if (R__likely(tag > 0)) {
   	void *varPtr = reinterpret_cast<unsigned char *>(to) + fVariantOffset;
   	CallConstructValueOn(*fSubfields[tag - 1], varPtr);
   	CallReadOn(*fSubfields[tag - 1], variantIndex, varPtr);
   	}

Additional context

We discussed a conservative approach to check exact identity of the type name.

[hahnjo]

As indicated, I think we should automatically support the first case (the variant just never has a value of that type). For the second case, we could either forbid it or put the variant into a valueless state.