feature - add generic Session.write(data, target) sink API over current file-specific writers

[dannymeijer]

Summary

RFC 004 currently ships Session.write_csv(...) and Session.write_parquet(...), but defers a generic sink entrypoint.

This issue tracks adding:

• Session.write(data, target)

without breaking the existing typed Session ownership and execution boundaries.

Scope

• Define a portable sink target envelope (target) for file and future sink kinds.
• Route all writes through Session-owned execution path.
• Keep typed, non-overlapping sink/planning/execution errors.
• Preserve existing write_csv/write_parquet as convenience APIs (or thin wrappers).

Out of scope

• Non-DataFusion backend implementation details (tracked separately).
• Full workflow/orchestration integration.

Acceptance criteria

• Session.write(data, target) exists and is documented.
• Existing writer APIs either delegate cleanly or are intentionally retained with documented relationship.
• InQL CI passes with coverage for generic sink routing and typed failures.