Support adding guards on gateway’s handleConnection method.

## I'm submitting a...

[ ] Regression 
[ ] Bug report
[x] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead post your question on Stack Overflow.

## Current behavior Currently gateway’s handleConnection method don’t support guards. ## Expected behavior It would be helpful set current user on socket after successfully connected. ## Minimal reproduction of the problem with instructions ## What is the motivation / use case for changing the behavior? ## Environment

Nest version: X.Y.Z

 
For Tooling issues:
- Node version: XX  
- Platform:  

Others: