format: add `constraint_expression` field to `CONSTRAINT_SCHEMA`

[lidavidm]

What feature or improvement would you like to see?

We expose information about constraints, but not their definition, which is accessible in some systems.

• MSSQL: https://learn.microsoft.com/en-us/sql/relational-databases/system-catalog-views/sys-check-constraints-transact-sql?view=sql-server-ver17
• PostgreSQL: Feature Request: [postgresql] Comprehensive Metadata Discovery for Data Development Tools #3971 (points 10, 11)