Add bucket/on_demand_migration/source_client.rs on top of the shared
remote S3 builder: HEAD, ranged streaming GET, ListObjectsV2 with
source-prefix mapping, GetObjectTagging and an admin probe. Every request
carries the x-rustfs-/x-minio-source-proxy-request anti-loop markers and
a RustFS-OnDemandMigration/<version> User-Agent suffix; SSE-C source
objects are rejected as unsupported. SourceError classifies SDK failures
(not found, access denied, throttled, timeout, connect, server error)
with retryability and a stable metrics label. Debug output redacts
credentials.
Refs rustfs/backlog#2149
overtrue
fc6f1f1f78