proto_caster's as shared library have broken symbol visibility

Both native and wrapped proto_casters use several functions from proto_cast_util.cc. Unfortunately, most of these functions are not actually usable, as the pybind11 default hidden visibility is propagated to these functions.

This can be verified by analyzing the generated object file:

> readelf -a -CW ./build/CMakeFiles/pybind11_native_proto_caster.dir/pybind11_protobuf/proto_cast_util.cc.o  | grep -E 'FUNC .*GLOBAL'   592: 0000000000001630    41 FUNC    GLOBAL HIDDEN   160 pybind11_protobuf::PyBytesAsStringView(pybind11::bytes)
   593: 0000000000001660     3 FUNC    GLOBAL HIDDEN   160 pybind11_protobuf::PyProtoGetCppMessagePointer(pybind11::handle)
   594: 0000000000001670   172 FUNC    GLOBAL HIDDEN   160 pybind11_protobuf::PyProtoDescriptorFullName[abi:cxx11](pybind11::handle)
   595: 0000000000001720   158 FUNC    GLOBAL HIDDEN   160 pybind11_protobuf::PyProtoHasMatchingFullName(pybind11::handle, google::protobuf::Descriptor const*)
   748: 0000000000001ed0  1090 FUNC    GLOBAL HIDDEN   160 pybind11_protobuf::PyProtoSerializePartialToString(pybind11::handle, bool)
   829: 00000000000029e0   457 FUNC    GLOBAL HIDDEN   160 pybind11_protobuf::CProtoCopyToPyProto(google::protobuf::Message*, pybind11::handle)
   839: 0000000000003790  1089 FUNC    GLOBAL HIDDEN   160 pybind11_protobuf::AllocateCProtoFromPythonSymbolDatabase(pybind11::handle, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)
   845: 0000000000003be0   104 FUNC    GLOBAL DEFAULT  160 pybind11_protobuf::InitializePybindProtoCastUtil()
   846: 0000000000003c50   274 FUNC    GLOBAL DEFAULT  160 pybind11_protobuf::ImportProtoDescriptorModule(google::protobuf::Descriptor const*)
   847: 00000000000041b0   201 FUNC    GLOBAL HIDDEN   160 pybind11_protobuf::GenericPyProtoCast(google::protobuf::Message*, pybind11::return_value_policy, pybind11::handle, bool)
   848: 0000000000004280     8 FUNC    GLOBAL HIDDEN   160 pybind11_protobuf::GenericProtoCast(google::protobuf::Message*, pybind11::return_value_policy, pybind11::handle, bool)

All method using a parameter from the pybind11:: namespace becomes "hidden", and when linking the object file into the shared library the HIDDEN symbols are omitted from the symbol table.

See https://pybind11.readthedocs.io/en/stable/faq.html#someclass-declared-with-greater-visibility-than-the-type-of-its-field-someclass-member-wattributes

google-or-tools e.g. patches the pybind11_protobuf build to create a static library (which is not affected by symbol visibility) to work around this problem:
https://github.com/google/or-tools/blob/2384738a951561bf905f3435651310841c309b4c/patches/pybind11_protobuf.patch#L32

Start with proto_cast_util.cc and the pybind11_protobuf build configuration, then reproduce the reported symbol visibility with the readelf command from the issue. Check the shared-library build and confirm that the functions used by native and wrapped proto_casters remain available in its symbol table.