CRUD API for table constraints

[kgodey]

Problem

The user might want to create a constraint on a table, and we should allow them to do so.

Proposed solution

Create a api/v0/tables/<id/constraints/ endpoint that allows users to create, read, update, or delete table constraints. For now, we should only support UNIQUE constraints, but please keep in mind that we may want to extend the API to other constraints in the future.

Additional context

• Design issue: Design for editing column properties and table constraints #324
• Postgres documentation on constraints