Fix severe warning from class loader in servo

[bijoua29]

Description

To fix "SEVERE WARNING" from class loader in moveit_servo, created the class loader as a member variable instead of creating it on the stack.

This fixes the following error:

[servo_node-20] Warning: class_loader.ClassLoader: SEVERE WARNING!!! Attempting to unload library while objects created by this loader exist in the heap! You should delete your objects before attempting to unload the library or destroying the ClassLoader. The library will NOT be unloaded.

[AndyZe]

Hi Bijou, long time no see. Thanks for fixing this, it's been annoying for too long.

I don't think it needs to be a unique_ptr? But I'll still approve the PR either way.

Suggested change

	// Plugin loader
	std::unique_ptr<pluginlib::ClassLoader<online_signal_smoothing::SmoothingBaseClass>> smoother_loader_;
	// Plugin loader
	pluginlib::ClassLoader<online_signal_smoothing::SmoothingBaseClass> smoother_loader_;

[AndyZe]

I pushed a small commit to satisfy the linter

[AndyZe]

Blocked pending #3567 I think

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (4fc5c82) to head (6becd9d).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3577   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files        180     181    +1     
  Lines      22601   22649   +48     
  Branches    3099    3108    +9     
=====================================
- Misses     22601   22649   +48     

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