There is a missing feature（notify-listen） about pg databases.

## Bug Report

**For English only**, other languages will not accept.

### Which version of ShardingSphere did you use?
5.5.1

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

### Expected behavior

There is 2 psql client named listener and notifier.
step 1: listener listen an issue

step 2: notifier send message to issue

step 3: listener execute any sql such as 'select 1;',will get the message from notifier

### Actual behavior
The proxy parsed the sql correctly, but there was no reaction;

### Reason analyze (If you can)
Maybe the proxy does not transparently transmit these network packets.

### Steps to reproduce the behavior, such as: SQL to execute, sharding rule configuration, when exception occur etc.
1. psql -h -p -U -d to login database;
2. listener : listen [any String such as test]
3. notifier: notify [test],'hello world'
4. listener : any sql,such as 'select 1;'

## Perhaps other databases support similar mechanisms

### Example codes for reproduce this issue (such as a github link).