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).

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.20%. Comparing base (e489b04) to head (64fe3a5).
⚠️ Report is 3 commits behind head on jazzy.

Files with missing lines	Patch %	Lines
moveit_ros/moveit_servo/src/servo.cpp	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            jazzy    #3598      +/-   ##
==========================================
- Coverage   46.21%   46.20%   -0.00%     
==========================================
  Files         718      718              
  Lines       62675    62672       -3     
  Branches     7589     7588       -1     
==========================================
- Hits        28957    28951       -6     
- Misses      33554    33558       +4     
+ Partials      164      163       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[MatthijsBurgh]

@AndyZe @nbbrooks please review and merge this one