build: find Protobuf using config mode search

[ashay]

This patch makes the Protobuf package mandatory in addition to forcing a
config mode search. The (default) module mode search looks for the
CMake-provided FindProtobuf.cmake file, but this file does not list
Abseil as a dependency, causing linker issues like the one below:

ld: Undefined symbols:
  absl::lts_20230802::log_internal::LogMessageFatal::LogMessageFatal(char const*, int, std::__1::basic_string_view<char, std::__1::char_traits<char>>), referenced from:
      google::protobuf::RepeatedPtrField<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>::TypeHandler::Type const& google::protobuf::internal::RepeatedPtrFieldBase::Get<google::protobuf::RepeatedPtrField<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>::TypeHandler>(int) const (.cold.1) in OnnxImporter.cpp.o

By forcing a config mode search, CMake looks for the file that is
installed as part of the protobuf package and which does contain the
Abseil dependency. This workaround is also mentioned in a GitHub issue
for Protobuf:
protocolbuffers/protobuf#12292 (comment).

[stellaraccident]

Ugh. I don't know how such a "simple" code generator can be such a disaster to get installed properly. Anyway, thanks for the fix.