Don't sort identity keys when creating unique indexes

[joseph-lozano]

Describe the bug
When creating an identity with multiple keys, unique_indexes should have the same key order as defined by the user.

To Reproduce
An identity with multiple keys not in alphabetical order:

identities do
  identity :unique_name_per_user, [:user_id, :name]
end

Run mix ash_postgres.generate_migrations

Verify that the generated migration has they keys on the unique index in a different order

Expected behavior
The generated index should have the same order
** Runtime

• elixir 1.16.2
• erlang 26.2.4
• Macos
• :ash, "~> 3.0"
• :ash_postgres, "~> 2.0"

Additional context
N/A