[Feature Request] Allow an optional search level in milvusautoindex

Milvus already exposes an AutoIndex search level through its
Go SDK.
VectorDBBench's Milvus AutoIndexConfig and milvusautoindex CLI do not expose
this parameter, so it cannot be set when benchmarking servers that implement it.

Add --level (YAML: level) from 1 to 10, forwarding it as search_params.params.level.
If omitted, leave search parameters unchanged. Index creation should not change.
The server determines how the level is used; this only adds client-side parameter support.

Example

Search an existing, loaded collection containing the selected dataset and an
AUTOINDEX index:

vectordbbench milvusautoindex \
  --uri http://localhost:19530 --collection-name VDBBench \
  --case-type Performance768D1M --level 2 --k 10 \
  --skip-drop-old --skip-load --skip-search-concurrent

Start with the Milvus AutoIndexConfig and milvusautoindex CLI entry point, then trace how search parameters are assembled. Add the optional CLI/YAML level value with the stated 1–10 range and verify that it is forwarded as search_params.params.level while omission leaves parameters unchanged. Use the supplied vectordbbench command against a loaded AUTOINDEX collection to check the behavior.