demand of scan by partial key

如Key为： url site ip xxx

可以指定Key中的某个子串进行san。如这里的数据以某个分隔符（空格）切分后，按照第x个列的值
进行scan。

No files, tests, or entry points are named. Start by locating the scan implementation and the key-encoding or key-comparison logic, then determine how a delimiter and selected column could affect scan bounds. Done means scans can select records by the requested key substring or column without changing existing full-key scan behavior, with coverage for the example key format.

Written by the indexing model from the issue text.