Optional skip validation for concurrent foreign key

What does this MR do?

Adds validate: option with a default value of true to the add_concurrent_foreign_key option.

If a column has been added to a table but no FK added, this allows us to:

• avoid the downtime that add_foreign_key(validate: false) requires
• skip validation that would error the migration, and perform it at a later time after all the data is fixed

Context: !21907 (comment 264456992)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

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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