C++ Warning fixes, werror on ci, C++17 compliance

[Wumpf]

What

• Fixes Build with warnings as errors on CI for C++ #2903
• Part of C++ SDK V1 #2919
• currate warning list for gcc/clang
• enable warnings as errors on CI, for simplicity it's always active on roundtrip tests
• enable standard set of warning settings on all C++ targets
• fix warnings in codegen'ed code and manually written code
• fix accidental C++20 (should have been caught in theory by warnings but it didn't trigger for unknown reason)

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested demo.rerun.io (if applicable)

• PR Build Summary
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[emilk]

🤨

[emilk]

fix accidental C++20 (should have been caught in theory by warnings but it didn't trigger for unknown reason)

Try -Wc99-extensions

[Wumpf]

-Wc99-extensions sadly still didn't catch

        rr_data_row c_data_row {
            .entity_path = entity_path,
            .num_instances = static_cast<uint32_t>(num_instances),
            .num_data_cells = static_cast<uint32_t>(num_data_cells),
            .data_cells = c_data_cells.get(),
        };

but left it in anyways

[Wumpf]

Remaining warning happens to be fixed in #2959. Not duplicating it in here, waiting for that instead!

[emilk]

Always nice to run on as many warnings as possible, especially for unsafe languages