feat: add python bindings for context store

[beinan]

Summary

• store context entries via the Rust ContextStore from Python bindings
• add a Pythonic Context API wrapper with dataframe/image normalization
• fix null embedding padding when writing to Arrow

Testing

• uv run pytest
• cargo test --manifest-path rust/lance-context/Cargo.toml
• cargo fmt --manifest-path rust/lance-context/Cargo.toml -- --check
• cargo clippy --manifest-path rust/lance-context/Cargo.toml --all-targets -- -D warnings
• ruff format --check python/
• ruff check python/
• pyright