feat: Allow ducklake_merge_adjacent_files to optionally run across schema version

[zhming0]

This solves #1048.
Fixes #536.

Summary

Adds an opt-in allow_cross_schema boolean parameter to ducklake_merge_adjacent_files (default false). When set, files written under different schema_versions are merged together into the current schema.
Fixes the limitation where ADD COLUMN / RENAME / etc. leave small files stranded on each side of a schema seam forever.

Safety considration

• Default behavior is unchanged, current test compaction_schema_version_per_table.test still passes.
• I suppose not all schema changes are compatible to each other, but I propose that we leave this decision to user to decide.

Usage

CALL ducklake_merge_adjacent_files('ducklake', 't', allow_cross_schema => true);