Enable loading a render engine plugin from the static plugin registry

[shameekganguly]

🎉 New feature

Summary

Extends RenderEngineManager to load a render engine plugin from the static plugin registry.

Also registered the default render engine plugins based on lookup in the plugin registry at runtime, in case one or more of the default render engines were statically linked in. This is required so that unloadEngine() can correctly unload the instantiated engine using the standard name, e.g. "ogre2". (Note: unloadEngine is called from the gz::sim::Sensors system). To make the plugin filename in the static plugin registry more obvious, I also added aliases for the existing render engine plugins with fully qualified names.

Test it

Added a test in the bazel build:

$ bazel test test:INTEGRATION_load_static_render_engine_plugin

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[iche033]

looks good to me