SQLAlchemy: Improve uniqueness polyfill to accept multiple columns (composite unique constraint / unique composite key)

[amotl]

About

GH-59 added a few utilities useful for additonal SQLAlchemy support, for example, a polyfill mechanism to emulate unique constraints.

Problem

The implementation can only handle unique constraints on single columns. In practice, we need more.

References

• Fix uniqueness constraint with .auth.db.models.SqlUser.username mlflow-cratedb#52 (review).

[amotl]

It has been implemented with GH-70.