feat: [3.0] add async snapshot export job APIs

issue: #44358
pr: #644

Backport the asynchronous snapshot export contract to the 3.0 branch so
Milvus 3.0 can return durable export job IDs and query job state,
progress, completion URI, and total exported bytes.

The source commits conflicted only in generated protobuf output and the
MsgType enum context. Preserve the 3.0 contracts, add the new message
type, and regenerate Go bindings from the resolved 3.0 proto sources.

Validation:
- make generate-proto
- repeat generation produced the same staged tree
- cd go-api && go test ./...
- git diff --cached --check

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

Author: weiliu1031