`set dist variable sql_show = 1` showed succeed but ineffective

## Bug Report

### Which version of ShardingSphere did you use?

master - 2da012130897dd302482e79c9c58424d0cab2c22

### Which project did you use? ShardingSphere-JDBC or ShardingSphere-Proxy?

ShardingSphere-Proxy

### Expected behavior

Enable `sql-show` or reject input.

### Actual behavior

```
bmsql_sharding=> set dist variable sql_show = 1;
SUCCESS

bmsql_sharding=> show dist variables;
variable_name | variable_value
---------------------------------------+-----------------
agent_plugins_enabled | true
cached_connections | 0
cdc_server_port | 33071
check_table_metadata_enabled | false
kernel_executor_size | 0
max_connections_size_per_query | 1
proxy_backend_query_fetch_size | 50
proxy_default_port | 3307
proxy_frontend_database_protocol_type |
proxy_frontend_executor_size | 4
proxy_frontend_flush_threshold | 128
proxy_frontend_max_connections | 0
proxy_frontend_ssl_cert_file |
proxy_frontend_ssl_cipher |
proxy_frontend_ssl_enabled | true
proxy_frontend_ssl_key_file |
proxy_frontend_ssl_version | TLSv1.2,TLSv1.3
proxy_hint_enabled | false
proxy_instance_type | Proxy
proxy_meta_data_collector_enabled | true
proxy_netty_backlog | 1024
sql_federation_type | NONE
sql_show | false
sql_simple | false
system_log_level | INFO
transaction_type | LOCAL
(26 rows)

```

Start by reproducing the reported SET DIST VARIABLE sql_show = 1 and SHOW DIST VARIABLES sequence in ShardingSphere-Proxy. Trace how the sql_show variable is handled; done means the setting enables sql-show or the input is rejected instead of returning SUCCESS.

Written by the indexing model from the issue text.