fix build against protobuf v34+

[r-burns]

Upstream protobuf changed StringViewArg from const std::string& to absl::string_view: protocolbuffers/protobuf@aeb5f2d

By using the same StringViewArg alias they use in their declarations, we can fix the build against protobuf v34+ without losing support for older versions.

We get new warnings, because this alias is deprecated, but I think that's better than bumping the required protobuf version unnecessarily. This approach can always be revisited if they fully drop StringViewArg in the future.

Tested against protobuf v32.1 and v34.1.

[Mizux]

missing include header to get the definition of StringViewArg

aka #include "google/protobuf/descriptor_database.h"
(which seems part of the public api since it is installed by dev package
e.g. https://pkgs.alpinelinux.org/contents?name=protobuf-dev&repo=main&branch=edge&arch=x86)

/home/runner/work/pybind11_protobuf/pybind11_protobuf/pybind11_protobuf/proto_cast_util.cc:364:9: error: ‘StringViewArg’ has not been declared
  364 |         StringViewArg filename
      |         ^~~~~~~~~~~~~

EDIT: seems more complicated since header is already included above...
https://github.com/r-burns/pybind11_protobuf/blob/ca65374f0e6caf8c4f2e34df8c9745103c8894d8/pybind11_protobuf/proto_cast_util.cc#L28

Please also note the using is protected: so you can't use DescriptorDatabase::StringViewArg outside...

[r-burns]

Weird, builds OK on my setup, are you on a slightly different protobuf version than me? The header is included above as you noted so I don't think it's that.

Please also note the using is protected: so you can't use DescriptorDatabase::StringViewArg outside...

I think the public inheritance on DescriptorDatabase here is what makes the StringViewArg using still available.