Fix two bugs:
1. Missing column is case sensitive, change the column name to lower case in FE for hive/iceberg/hudi
2. Iceberg use custom method to encode special characters in column name. Decode the column name to match the right column in parquet reader.
sort out the test cases of external table.
After modify, there are 2 directories:
1. `external_table_p0`: all p0 cases of external tables: hive, es, jdbc and tvf
2. `external_table_p2`: all p2 cases of external tables: hive, es, mysql, pg, iceberg and tvf
So that we can run it with one line command like:
```
sh run-regression-test.sh --run -d external_table_p0,external_table_p2
```
