storage: register tier backends at the binary composition root (#9989)

The s3 and rclone tiered-storage backends were registered via blank imports
in weed/storage (volume_tier.go and volume_info/volume_info.go). That forced
every library consumer of weed/storage -- weed/shell, and through it external
tools -- to link aws-sdk-go and, under the rclone build tag, the full rclone
backend set and its cloud-storage SDKs, even though those consumers never tier
volumes.

Move the registrations into a new weed/storage/backend/all aggregator and
blank-import it once from weed/command, the binary's composition root. The weed
binary still registers both backends; weed/storage and its library consumers no
longer pull the backend SDKs into their dependency graph.

weed/command/backends.go

@@ -0,0 +1,5 @@
+package command
+
+// Register the tiered-storage backends here, at the binary's composition root,
+// so library consumers of weed/storage don't pull the backend SDKs.
+import _ "github.com/seaweedfs/seaweedfs/weed/storage/backend/all"