pyiceberg-core: add build-pyiceberg-core.yml for riscv64 wheels

[luhenry]

• Package: pyiceberg-core
• Version: 0.10.1
• Source: https://github.com/apache/iceberg-rust
• Docs: https://rust.iceberg.apache.org/

Compiles the Rust-based Iceberg core (the bindings/python crate of the apache/iceberg-rust Cargo workspace) into the abi3 extension PyIceberg optionally loads. Upstream publishes no riscv64 wheel.

Mirrors upstream's release_python.yml.

Differs from upstream

• Builds and tests on cp312, not the cp310 abi3 floor - pypi.riseproject.dev has no riscv64 pyarrow/pyiceberg wheel below cp312.
• RUSTUP_TOOLCHAIN=stable - the checkout's rust-toolchain.toml pins a nightly for lint only; the sdist excludes it, the checkout does not.

Testing

• test_transform.py, same as upstream.
• test_datafusion_table_provider.py and test_huggingface_and_cdc.py dropped - both unconditionally import datafusion, which has no riscv64 wheel anywhere.
• test_manifest.py dropped - it imports pyiceberg.io.pyarrow, which unconditionally imports pyarrow._s3fs; our riscv64 pyarrow ships ARROW_S3=OFF.

License: OK

Built on cp312; 11 passed.