feat: expose ducklake tables size to Postgres

[YuweiXiao]

Description

Support \dt+ and pg_relation_size for ducklake table.

[KarinaMilet]

Please assign this to me.

[qsliu2017]

Investigation: pg_relation_size / \dt+ for DuckLake tables

PostgreSQL's pg_relation_size(), pg_table_size(), and pg_total_relation_size() (which \dt+ uses) all call calculate_relation_size() in dbsize.c, which directly stat()s physical files on disk. They do not call the table AM's relation_size callback. Since DuckLake tables store data in parquet files (and inlined PG tables) rather than in PG's standard relation files, these functions always return 0.

This is a known PostgreSQL limitation affecting all custom table AMs:

• Citus Columnar works around it with a custom citus_table_size() function
• OrioleDB requires patches to PostgreSQL core
• There is an open pgsql-hackers discussion about making dbsize.c use the table AM API, plus a proposed hook patch -- neither has been merged

What we can do

1. Custom function ducklake.table_size(regclass) -- queries ducklake_data_file.file_size_bytes, ducklake_delete_file.file_size_bytes, and inlined data table sizes. Works correctly but \dt+ still shows 0.
2. Upstream contribution -- submit or support a patch to PG core that routes pg_table_size() through the AM's relation_size callback for non-heap AMs.

The DuckLake table AM's relation_size callback can be implemented to return the correct size (useful for VACUUM internals and future PG versions), but it won't affect pg_relation_size() or \dt+ until PostgreSQL core is updated.