feat: add multimodal content serialization

[beinan]

Summary

• add multimodal serde helpers for image and Arrow IPC dataframe serialization
• add unit tests for image/dataframe serialization
• ignore BSP/IDE workspace directories in git

Testing

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