1. `std::string` to `std::wstring` conversion only supports ASCII
characters. For non-ASCII characters, we need to use
`StringUtil::string_to_wstring`
2. Fix index_tool check_terms_stats_v2 and add field info to print
pick from master #36321
Normally we write the separate index files to disk before we merge the index files into an idx compound file.
In high-frequency load scenarios, disk IO can become a bottleneck.
In order to reduce the pressure on the disk, we write the standalone index file to the RAM directory for the first time, and then write it to the disk when merging it into a composite file.
Add config `index_inverted_index_by_ram_dir_enable`, default is `false`.
