Geo: Avoid rereplicating Blobs that already exist

From #209813 (closed) and &7180 (comment 821509584) and &2153 (comment 837863715):

Problem: Restore a GitLab backup onto a new Geo site. Backfill could be much faster.

Problem: Failover and demote the primary. Backfill could be much faster.

Proposal

• Require the Verification feature
• If a blob registry is brand new (maybe like pending? && verification_pending?), and a blob already exists in the expected location, then just mark it synced
• Make sure that rereplication actually does occur, when verification fails => I did test this locally in !96935 (merged) and it worked
• Add tests in shared examples for verified SSF data types

To do

• Doc update is in !140276 (merged)