Fix severe warning from class loader in servo (backport #3577)

[mergify]

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.

---

This is an automatic backport of pull request #3577 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 92654dd has failed:

On branch mergify/bp/humble/pr-3577
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit 92654dd1b.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   moveit_ros/moveit_servo/include/moveit_servo/servo.hpp
	both modified:   moveit_ros/moveit_servo/src/servo.cpp

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[MatthijsBurgh]

@AndyZe please resolve the conflicts

[rhaschke]

The PR doesn't apply to Humble, because the refactoring (#2224) isn't part of the Humble branch.
On Humble, the smoothing loader is already persistent:

moveit2/moveit_ros/moveit_servo/include/moveit_servo/servo_calcs.h

Line 359 in 2b2030d

pluginlib::ClassLoader<online_signal_smoothing::SmoothingBaseClass> smoothing_loader_;