Add knowlege of constraints to internal schema representation

[andrew-farries]

Constraints (initially CHECK and FOREIGN KEY) should be represented in pg-roll's internal schema representation.

Also, UNIQUE constraints should be part of the internal schema representation too.

Having constraints represented in the internal schema would enable:

• Better validation of operations involving constraints:
  • Ensure that a constraint of the same name does not exist when adding a new constraint.
  • Ensure that the constraint exists when dropping a constraint.
• Not having to specify the name of the column on which the constraint is defined when dropping a constraint (and the confusion that occurs when the wrong column is specified)

Tasks

Beta Give feedback

1. Validate constraint exists before dropping (Ensure constraint existence as part of validation for the drop constraint operation #252)
2. Ensure constraint of same name doesn't exist when adding an FK constraint
3. Ensure constraint of same name doesn't exist when adding a CHECK constraint
4. Ensure constraint of same name doesn't exist when adding a UNIQUE constraint
5. BREAKING CHANGE: no longer require Column field when dropping a constraint