Initialize services in ViewAngle constructor

[caguero]

🦟 Bug fix

Summary

When loading the ViewAngle GUI plugin, the terminal shows the following message:

caguero@cold:~$ gz sim /home/caguero/garden_ws/install/share/gz/gz-sim7/worlds/minimal_scene.sdf
Service [] is not valid.

The reason for this is that we're trying to call Request() before the appropriate service name has been initialized. Before this patch, the services are initialized in LoadConfig(). We're now initializing the services in the constructor.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
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• All tests passed (See test coverage)
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