feat(admin): add a gateway key inventory for the S3 stack switch (#7785)

GET /rustfs/admin/v3/gateway-key-inventory (admin:InspectData) lists
every stored object key the RustFS S3 gateway would refuse on every
operation, so an operator can copy those objects to a safe key before
switching stacks. The rules mirror the gateway key floor; . and ..
segments never appear because ecstore refuses them on every write.

Refs rustfs/gateway#754