feat: support JuiceFS as Block Service

## Feature Request
**Is your feature request related to a problem? Please describe:**
JuiceFS provides a high-performance, scalable POSIX filesystem that can serve as an alternative storage backend to HDFS for Pegasus cold backup scenarios.

**Describe the feature you'd like:**
Add support for JuiceFS as a block service backend for cold backups and data storage.

**Implementation Details**

- New provider juicefs_service registered using existing hdfs_service as underlying implementation
- Patches RocksDB to allow HDFS protocol access to JuiceFS storage

Start by tracing the existing hdfs_service implementation and the block-service registration path. Confirm how JuiceFS should use that implementation, then inspect the RocksDB integration mentioned in the issue. Done means cold backups and data storage can use a registered juicefs_service backend with HDFS protocol access.

Written by the indexing model from the issue text.