Feat(utils): psql unique index instead of constraint (#6386)

**What**
- always return an index expression, also for unique constraints

**why**
- constraints can't be partial, meaning `UNIQUE ... WHERE` is not possible with constraints
- constraints are indicies under the hood so it doesn't change the behavior of the system when we're not using constraint specific features but just using them for `UNIQUE`

.changeset/hot-coins-itch.md

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+---
+"@medusajs/utils": patch
+---
+
+feat(utils): make psql index util return index instead of constraint for unique indicies becuase partial constraints don't exist :'(