Use ign-plugin

[chapulina]

Ignition Sensors is using ign-common's plugin loader, but that's deprecated. It should start using ign-plugin instead:

https://github.com/ignitionrobotics/ign-plugin

[ahcorde]

• ign-sensor Replace ign common plugin loader with ign-plugin #38
• ign-gazebo Use the symbols if they are already loaded gz-sim#304

[chapulina]

I build on top of @ahcorde 's work with some extra suggestions on:

• Remove plugin interface and support custom sensors #90
• Accept null dlhandle and support custom dlopen flags gz-plugin#35

[chapulina]

Instead of using ign-plugin, we completely removed the plugin interface on #90.