Add batched background migration to backfill X.509 tables project_id

What does this MR do and why?

Contributes to #553480 (closed)

Backfills project_id for x509_certificates and x509_issuers tables by processing x509_commit_signatures records in batches.

References

• Sharding Key Implementation for `x509_certifica... (#553480 - closed)
• Sharding Key Implementation for `x509_issuers` ... (#553481 - closed)

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Related to #553481 (closed)