Add repro test for concurrent index ordering with dependent foreign keys

[jechol]

Adds a failing regression test for a migration ordering issue with concurrent_indexes: true.

The repro covers a resource with a non-primary-key identity on :code and another resource with a belongs_to that references that identity via destination_attribute(:code).

The generated migrations place the foreign key migration before the concurrent unique index migration, even though the referenced column needs the unique index before the foreign key can be added.

A standalone repro project is available at:
https://github.com/jechol/ash-postgres-concurrent-indexes-repro

This PR is intended as a minimal repro only.

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