Aurora DSQL speaks a PostgreSQL-compatible wire protocol, but uses IAM
authentication and has DDL limits that differ from PostgreSQL.
Serverless deployments select it with a dsql:// DATABASE_URL while the
Diesel layer continues to use the PostgreSQL backend.
Add a dsql feature and classify dsql:// URLs as PostgreSQL. Convert them
to libpq URLs with cached IAM authentication tokens. Route DSQL
databases through dedicated migrations rather than PostgreSQL
migrations.
Keep deployed DSQL migration versions and add DSQL migrations for the
upstream schema changes needed by this rebase. Split newer migrations
into transactionless, single-DDL steps for Aurora DSQL.
Co-authored-by: Copilot <copilot@github.com>