Add unique validation to additional pack purchase_xid

What does this MR do?

In !62393 (merged) we added a new (currently unused) table for tracking CI Minute provisioning.

This table will primarily to be used for two things:

1. tracking purchases of additional CI minutes
2. managing the provisioning of those minutes (expiry, usage, etc)

The second point introduces a slight complexity, in that we discovered our self-managed customers may also be using the concept of CI minute limits / additional minutes, to prevent over-use on their own runners.

This means that we need to support both GitLab.com and self-managed instances, so the table was originally created with no unique constraints or indices.

This MR attempts to add a unique index solely for GitLab.com so that we can attempt to ensure that entries are unique scoped to namespace and purchase ID.

I've opted not to add a unique index for Gitlab.com only, as diverging the GL database away from SM seems high risk. Instead we'll just use a standard rails validation and accept the increased risk of duplicate entries due to race conditions (unlikely anyway, as it will not be used via a public API)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
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• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

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Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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