GraphQL mutation to retry failed reassigments

What does this MR do and why?

This adds a GraphQL mutation to allow users to retry failed placeholder reassignments via the API using a new service implemented in !227954 (merged).

Reassigning placeholder records is an expensive operation, the retry reassignment service prevents users from repeatedly retrying a source user that continues to fail by tracking the number of retry attempts in redis on each successful retry. Users are allowed to retry a failed source user up to 3 times within 16 hours before receiving an error containing the time time they must wait before trying again.

References

• Placeholder user mapping docs: https://docs.gitlab.com/user/import/mapping/#bypass-confirmation-when-reassigning-placeholder-users
• Related to Unable to retry or recover from FAILED status d... (#589777)

How to set up and validate locally

Follow the validation steps in !227954 (merged) to create a source user and make it fail, but use the new mutation to retry the failed reassignment instead of the console:

mutation {
  importSourceUserRetryFailedReassignment(input: { id: "gid://gitlab/Import::SourceUser/<SOURCE_USER_ID>" }) {
    importSourceUser {
      id
      status
      reassignToUser {
        id
        username
      }
    }
    errors
  }
}

Validate:

• A source user can be successfully retried
• A source user that fails on retry can only be retried 3 times in 16 hours (RETRY_COOLDOWN can be lowered from 16 hours to a few minutes for testing)

MR acceptance checklist

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Related to #589777