Add query_language to lotus_queries.

The column records the language identifier (family:dialect, e.g. sql:postgres) that a stored query was written for. It is nullable: NULL means "derive the language from the data source's adapter", which is the behaviour of every row that existed before this migration.

There is no backfill. A backfill would have to read runtime configuration and resolve adapters from inside a schema migration, and NULL already means exactly what a backfill would compute.