feat(datafusion): support SHOW CREATE TABLE for external engines

[plusplusjiajia]

Purpose

Support SHOW CREATE TABLE on external data tables served by a registered table engine:

SHOW CREATE TABLE lake.db.events;

The definition comes from the engine's provider (TableProvider::get_table_definition) and is returned unchanged in the existing four columns.

Brief change log

• SQLContext classifies the target with load_table. Paimon tables keep the generated DDL; external tables are resolved through the catalog provider, which applies the existing session option and query-auth checks.
• Only the target table is resolved; the catalog is not enumerated through information_schema.views.
• An unregistered engine, a missing table and engine errors are reported. A provider without a definition, or with a blank one, is reported as unsupported instead of returning Paimon DDL or an empty result.
• Update the DataFusion README.

Tests

• External tables return the engine DDL unchanged, including quoted table names and tables in other databases.
• Missing engine, missing or blank definition, missing table and engine errors are reported.
• A failing unrelated table does not affect the target table.
• Session scan options and query-auth are checked before the engine is called.
• A temporary object shadowing an external table is described without calling the engine; Paimon tables keep their DDL.