Enable protobuf support

[Alextopher]

Submitting on behalf of the photonvision project.

In order to seamlessly support object detection through OpenCV DNN module we ask that wpilib enable protobuf support. Protobuf support is required to load DNN models in the .onnx file format.

While waiting for 2025 versions we ask that a 2024 v4.8.0-5 could be released with this change in the meantime.

CC @mcm001

[PeterJohnson]

This is not a trivial change. We can't just enable it, because it will result in symbol conflicts between the system protobuf picked up by the cmake build and the protobuf library integrated into wpilib. We'd either be introducing a circular build dependency on allwpilib when upgrading opencv, or require us to move the protobuf build to thirdparty-protobuf and have this and the allwpilib build pull down those libraries.

[mcm001]

It looks like Opencv has a BUILD_PROTOBUF flag, which will make opencv build a libprotobuf static library, yeah.

[PeterJohnson]

That doesn't help, as in static libraries the symbols are still exposed to people linking against opencv.

[mcm001]

Yeah sorry poorly phrased, I was trying to say that. If we were able to make sure opencv shared libraries did not import/export protobuf symbols would we be OK?

[Alextopher]

What about

WITH_PROTOBUF=ON
BUILD_PROTOBUF=OFF

Edit: This doesn't solve any problems as it would require pulling a thirdparty-protobuf dependency in.

[PeterJohnson]

We build and use both shared and static libraries, so we need both to work, and it’s impossible to hide/embed the protobuf static library symbols.