issue: https://github.com/milvus-io/milvus/issues/44399
This PR implements STL_SORT for VARCHAR data type for both RAM and MMAP
mode.
The general idea is that we deduplicate field values and maintains a
posting list for each unique value.
The serialization format of the index is:
```
[unique_count][string_offsets][string_data][post_list_offsets][post_list_data][magic_code]
string_offsets: array of offsets into string_data section
string_data: str_len1, str1, str_len2, str2, ...
post_list_offsets: array of offsets into post_list_data section
post_list_data: post_list_len1, row_id1, row_id2, ..., post_list_len2, row_id1, row_id2, ...
```
---------
Signed-off-by: SpadeA <tangchenjie1210@gmail.com>
6077178553