Fix rviz crash issue if run later

[mini-1235]

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Basic Info

Info	Please fill out this column
Ticket(s) this addresses	#4754
Primary OS tested on	Ubuntu 24.04
Robotic platform tested on	turtlebot 3 simulation
Does this PR contain AI generated software?	No

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Description of contribution in a few bullet points

On my machine, this not only stuck in getting parameter of planner, but also other plugins like controller, goal checker and so on. Thus, I did the following to solve this issue:

• Open a new thread to load plugins, to ensure that get_parameter() would not block the main thread
• Consequently, this new thread can also handle the case we start rviz first, and bringup later

Description of documentation updates required from your changes

None

Description of how this change was tested

Extensively tested on tb3 simulation

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Future work that may be required in bullet points

On the original implementation, the combo box will appear blank if it cannot get parameter from service, I now put it a "default" on those labels, I think this will be better (otherwise users will click on an empty label).

I am not really familiar with QT, not sure if there is any problem in my implementation :)

Currently only tested on Jazzy, but I could build new images and test it on different distributions

For Maintainers:

• Check that any new parameters added are updated in docs.nav2.org
• Check that any significant change is added to the migration guide
• Check that any new features OR changes to existing behaviors are reflected in the tuning guide
• Check that any new functions have Doxygen added
• Check that any new features have test coverage
• Check that any new plugins is added to the plugins page
• If BT Node, Additionally: add to BT's XML index of nodes for groot, BT package's readme table, and BT library lists

[mergify]

@mini-1235, all pull requests must be targeted towards the main development branch.
Once merged into main, it is possible to backport to @jazzy, but it must be in main
to have these changes reflected into new distributions.