onnxruntime: trying to use onnx with shared=True

[franramirez688]

After submitting #29205, ORT now compiles with onnx:shared=True, but it raises another bug when something links with the library:

WARNING: All log messages before absl::InitializeLog() is called are written to STDERR
E0000 00:00:1766751100.447597 5559872 descriptor_database.cc:678] File already exists in database: onnx/onnx-ml.proto
F0000 00:00:1766751100.448821 5559872 descriptor.cc:2518] Check failed: GeneratedDatabase()->Add(encoded_file_descriptor, size)
*** Check failure stack trace: ***
    @        0x10264a488  absl::lts_20250814::log_internal::LogMessage::Flush()
    @        0x1030ca854  google::protobuf::DescriptorPool::InternalAddGeneratedFile()
    @        0x10316c928  google::protobuf::internal::AddDescriptors()
    @        0x19ceaacb0  ___ZNK5dyld46Loader25findAndRunAllInitializersERNS_12RuntimeStateE_block_invoke
    @        0x19cee8730  ___ZNK5dyld313MachOAnalyzer18forEachInitializerER11DiagnosticsRKNS0_15VMAddrConverterEU13block_pointerFvjEPKv_block_invoke.204
    @        0x19cf07540  ___ZNK6mach_o6Header14forEachSectionEU13block_pointerFvRKNS0_11SectionInfoERbE_block_invoke
    @        0x19cf04164  mach_o::Header::forEachLoadCommand()
    @        0x19cf059fc  mach_o::Header::forEachSection()
    @        0x19cee8220  dyld3::MachOAnalyzer::forEachInitializer()
    @        0x19ceaaa68  dyld4::Loader::findAndRunAllInitializers()
    @        0x19ceb28b0  dyld4::JustInTimeLoader::runInitializers()
    @        0x19ceab214  dyld4::Loader::runInitializersBottomUp()
    @        0x19ceab1b4  dyld4::Loader::runInitializersBottomUp()
    @        0x19ceafe50  dyld4::Loader::runInitializersBottomUpPlusUpwardLinks()::$_0::operator()()
    @        0x19ceab530  dyld4::Loader::runInitializersBottomUpPlusUpwardLinks()
    @        0x19cecd04c  dyld4::APIs::runAllInitializersForMain()
    @        0x19ce8f158  dyld4::prepare()
    @        0x19ce8dd04  start
/bin/sh: line 1: 28703 Abort trap: 6           ./test_package

Reviewing the ONNX implementation, onnx is linking with onnx_proto_object too (https://github.com/onnx/onnx/blob/v1.20.0/CMakeLists.txt#L459), so I think that we can remove onnx_proto as an external dependency to avoid that error.

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