enhance: add CubeFS as supported cloud storage provider (#50711)

## What
Add CubeFS as a supported `cloudProvider` option in Milvus object
storage configuration.

## Changes
- Add `CloudProviderCubeFS = "cubefs"` constant to
`pkg/objectstorage/util.go`
- Add CubeFS case in `NewMinioClient` switch: forces path-style bucket
lookup (`BucketLookupPath`) and static V4 credentials (IAM not supported
by CubeFS ObjectNode)
- Document `cubefs` as a valid `cloudProvider` value in
`configs/milvus.yaml`
- Add integration test `TestNewMinioClientCubeFS` (skipped unless
`CLOUD_PROVIDER=cubefs` env is set, consistent with all other provider
tests)

## Why path-style?
CubeFS ObjectNode requires `S3ForcePathStyle` — it does not support
virtual-host style bucket addressing (`bucket.host/key`).

issue: #26189

Signed-off-by: Ayush KAshyap <kashyap11ayush02@gmail.com>

configs/milvus.yaml

@@ -148,6 +148,7 @@ minio:
   # You can use "aliyun" for other cloud provider uses virtual host style bucket
   # You can use "gcpnative" for the Google Cloud Platform provider. Uses service account credentials
   # for authentication.
+  # CubeFS is S3-compatible and works with cloudProvider: aws and useVirtualHost: false (no IAM support).
   # When useIAM enabled, only "aws", "gcp", "aliyun" is supported for now
   cloudProvider: aws
   # The JSON content contains the gcs service account credentials.