no-mercure flag to disable mercure integration

**Description**  

Mercure is a killer feature but it depends on the project you are working one.
For a prototype or a side project it could be overkill to use it at the first time. Instead of cleaning the generated code I propose to add a simple flag to not generate the code related to Mercure.

**Example**  
```bash
client-generator --no-mercure
```