Modify constraints API to support creating foreign key constraints

[seancolsen]

Current behavior

• I can't create FK constraints.
• I can create a unique constraint with a POST to /tables/{table_id}/constraints/ and type: "unique".

Desired behavior

• Creating FK constraints should function similarly to creating unique constraints.

Notes

• The API for creating unique constraints is currently in flux due to Modify constraints API to use column id instead of column name #898. We should be sure to follow the same pattern for FK constraints, which will likely mean requiring the API request to specify all tables and columns by id instead of name.
• We don't plan to ever support updating FK constraints. If the user wants to change the constraint, they'll need to drop it and add a new one.
• If it is fairly quick to implement, the API should support as many of the following features as it can, even though there's no UI for them yet. If you do not support these in your PR, please design the API in a way that can be easily extended to support them in the future without breaking changes. It would also be helpful to create separate placeholder issues to track any issues not handled by the PR.
  • Creating deferred FK constraints
  • Setting the ON DELETE attribute to any of the five possible actions.
  • Setting the ON UPDATE attribute to any of the five possible actions.
  • Creating multi-column FKs, including the ability to pass MATCH FULL.

Additional context

• Blocked by API definition in Modify constraints API to list foreign key constraints #943
• Foreign Key Spec -- Scenario 1
• Postgres docs for Constraints
• Postgres docs for SET CONSTRAINTS
• The UI only supports single-column FKs, but we might want to add support for creating multi-column FKs in the future (discussion).

[seancolsen]

@kgodey I'm assigning this to you to help answer the above questions and loop others in as needed.

[kgodey]

@seancolsen I've updated the ticket description to resolve questions. I'm going to mark the ticket as blocked until #898 is resolved and the API for constraints stabilizes.

I'm also going to say this is blocked by #943 since that will define the foreign key constraint response.