Add shard routing types and the SplitShard message type for online shard split

Add the shared types used by Milvus online shard split of namespace
(multi-tenant) collections:

- schema.proto: RoutingMode (Hash/Range), ShardState, and CollectionShardInfo.
  Each shard carries its routing predicate via a oneof: RangeRouting (a list of
  byte-comparable [lower, upper) ranges) or HashRouting (a list of hash buckets).
  A shard may own more than one piece (e.g. the cold remainder after carving a
  hot tenant out of the middle), so each variant is a list; the lists of all
  shards tile the key space.
- milvus.proto: DescribeCollectionResponse gains shard_infos (parallel to
  virtual_channel_names) and routing_mode.
- common.proto: MsgType_SplitShard, the legacy msgstream type the SplitShard WAL
  message maps to so the flusher can consume it and seal the fenced segments.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Cai Zhang <cai.zhang@zilliz.com>

Author: xiaocai2333