[CH] Support convert `Char` type of HiveTable to `FixedString` in ClickHouse backend

### Description

Now that the char/varchar type of hive table are always convert to string type in clickhouse backend. And the char type in hive table means the field has fixed bytes, which is suitable to convert to `FixedString` type in clickhouse.

And in some suitation, the fixed string type can have better performance that the string type in clickhouse. such as the querys contains `order by`.

Here we defined 2 tables, has the same data, the first one is defined as `test_tbl1(id UInt64, d String)`, and the second is defined as `test_tbl2(id UInt64, d FixedString(20))`, and we do the query `select * from test_tbl order by d limit 100 settings max_threads=1` to compare the performance

In the first table, the result show as below

In the second table, the result show as below

and we can see that abort 20% ~ 30% performance improve when the table field is defined as `FixedString` type.

No files or tests are named. Start by locating the ClickHouse backend's Hive Char/varchar type-conversion path and review how fixed byte lengths are represented; done means Hive Char fields map to ClickHouse FixedString while varchar and other existing mappings remain unchanged, with coverage for the conversion.

Written by the indexing model from the issue text.