[Improvement] Support load indexes on multiple columns

### What would you like to be improved?

1. create the following table in mysql
```
CREATE TABLE test (
id INT NOT NULL,
last_name CHAR(30) NOT NULL,
first_name CHAR(30) NOT NULL,
PRIMARY KEY (id),
INDEX name (last_name,first_name)
);
```
2.load the table in gravitino
there's only PRIMARY KEY indexes

It's better to support load another indexes

### How should we improve?

_No response_

Start by reproducing the MySQL table definition and loading it into Gravitino, then trace the existing path that loads only PRIMARY KEY indexes. Check how a composite index on last_name and first_name is represented after loading, and verify that the non-primary index is preserved with both columns and its name.

Written by the indexing model from the issue text.