Switch nav2_rviz_plugins to modern CMake idioms.

[clalancette]

In addition, make some fixes to the code to more appropriately hold on to the Node object (there were warnings from the compiler without these fixes).

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Basic Info

Info	Please fill out this column
Ticket(s) this addresses	Follow-up to #4357
Primary OS tested on	Ubuntu 24.04
Robotic platform tested on	N/A
Does this PR contain AI generated software?	No

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Description of contribution in a few bullet points

Change nav2_rviz_plugins to use modern CMake idioms:

1. Make it a "nav2_package". This ended up adding in additional warnings, which then required some code changes to fix. Thus, there are some additional code changes in here to more appropriately hold onto weak pointers.
2. Switch from ament_target_dependencies to target_link_libraries.
3. Export the target so downstream CMake consumers can use it.
4. Push the include directory down one level, which is best practice since Humble.

Description of documentation updates required from your changes

None needed.

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Future work that may be required in bullet points

This is part of a larger series to switch Navigation2 to use modern CMake idioms. There will be follow-up PRs converting more of the packages.

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

[SteveMacenski]

For all: Some kind of log, even if cout, that this is the case would be nice (for all instances)

[clalancette]

Good call. Fixed in 864a450.