Add lint for user defined fkeys refing non-pkey unique constraints in auth schema

[olirice]

Adds a lint that check for user defined foreign keys that reference unique constraints in the auth schema.

• Referencing auth table pkeys is supported, referencing unique keys is not
• unique keys are scheduled for removal so we need these to be cleaned up to ensure a smooth rollout of future auth versions

[hf]

Looks good!