feat: Add snapshot API definitions for backup and restore

issue: milvus-io/milvus#44358
Add snapshot-related protobuf definitions and RPC services:
- Add snapshot privilege definitions in common.proto
- Add snapshot RPC services in milvus.proto
- Add snapshot.proto with request/response messages
- Update proto generation script to include snapshot files

This provides the foundation for implementing snapshot-based backup and restore
functionality with proper privilege control.

Signed-off-by: Wei Liu <wei.liu@zilliz.com>

Author: weiliu1031