feat: Add SQL write support (INSERT INTO) for DuckLake catalogs

[shefeek-jinnah]

This extends the write support introduced in #46 to work with DataFusion's
standard SQL interface, enabling INSERT INTO statements.

Changes:

• Add DuckLakeCatalog::with_writer() constructor to enable write operations
• Implement TableProvider::insert_into() for INSERT INTO statements
• Add DuckLakeInsertExec execution plan that writes to Parquet and registers
  files in the catalog metadata
• Propagate MetadataWriter through catalog -> schema -> table hierarchy
• Add SchemaProvider::register_table() scaffolding for future CTAS support

Supported SQL operations:

• INSERT INTO ... SELECT
• INSERT INTO ... VALUES