ShardingSphere-Proxy supports setting character encoding?

In some case, I want to set character-set when connect to shardingsphere-proxy by mysql client like the following command

```sql
mysql -h -P -u xxx -p --default-character-set=gbk
```
but shardingsphere-proxy not support this.

```
mysql> show variables like 'character%';
+--------------------------+-----------------------------------------------------------+
| Variable_name | Value |
+--------------------------+-----------------------------------------------------------+
| character_set_client | utf8 |
| character_set_connection | utf8 |
| character_set_database | latin1 |
| character_set_filesystem | binary |
| character_set_results | |
| character_set_server | latin1 |
| character_set_system | utf8 |
| character_sets_dir | /usr/local/mysql-5.7.30-macos10.14-x86_64/share/charsets/ |
+--------------------------+-----------------------------------------------------------+
8 rows in set (0.04 sec)

```

I use same command connect to mysql, it is work.
```
mysql -hlocalhost -P3306 -uroot -p --default-character-set=gbk
Enter password:

mysql> show variables like 'character%';
+--------------------------+-----------------------------------------------------------+
| Variable_name | Value |
+--------------------------+-----------------------------------------------------------+
| character_set_client | gbk |
| character_set_connection | gbk |
| character_set_database | latin1 |
| character_set_filesystem | binary |
| character_set_results | gbk |
| character_set_server | latin1 |
| character_set_system | utf8 |
| character_sets_dir | /usr/local/mysql-5.7.30-macos10.14-x86_64/share/charsets/ |
+--------------------------+-----------------------------------------------------------+
8 rows in set (0.00 sec)

```

I think we can enhance this feature.

Start by reproducing the reported connection with the MySQL client using --default-character-set=gbk, then compare SHOW VARIABLES LIKE 'character%' through ShardingSphere-Proxy and MySQL. Done means the proxy accepts the requested character set and reports matching client, connection, and results settings.

Written by the indexing model from the issue text.