refactor(ecstore): shared remote S3 client builder and ODM source client (#7067)

* refactor(ecstore): extract shared remote S3 client builder

Move the aws_sdk_s3 client construction out of bucket_target_sys into
bucket/remote_s3_client.rs: endpoint assembly, credential provider,
path-style selection, custom CA / skip-TLS transports and the outbound
SSRF gate now build from a neutral RemoteS3EndpointSpec so replication
targets and the upcoming on-demand migration source client share one
policy. Replication builds its client through From<&BucketTarget>; the
gate keeps its relaxed semantics (private allowed, loopback only behind
RUSTFS_REPLICATION_ALLOW_LOOPBACK_TARGET) verbatim. The builder also
gains optional connect/read timeouts and a User-Agent suffix
interceptor, both unset for replication.

Refs rustfs/backlog#2149

* feat(ecstore): add on-demand migration SourceClient

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

* docs(operations): point outbound policy at shared remote S3 client builder

scripts/error-other-format-baseline.txt

@@ -5,7 +5,6 @@
 # scripts/check_error_other_format_ratchet.sh --update-baseline. A PR that
 # raises a count or adds a file is introducing a new quorum-bucketing hazard
 # and must carry an explicit exemption rationale in its description.
-2|crates/ecstore/src/bucket/bucket_target_sys.rs
 4|crates/ecstore/src/bucket/lifecycle/bucket_lifecycle_ops.rs
 3|crates/ecstore/src/bucket/lifecycle/durable_namespace.rs
 2|crates/ecstore/src/bucket/lifecycle/metadata_boundary.rs