Add support for loading System plugin from static plugin registry

[shameekganguly]

🎉 New feature

Summary

Gz Plugin added support for loading plugins from statically linked plugin libraries in gazebosim/gz-plugin@a49b9f4.
This PR builds on top of this feature to enable System plugins to be loaded from the static plugin registry.

To indicate that a plugin should be loaded from the static registry, users can specify the sdf plugin@filename attribute as static://<fully qualified plugin class name> or static://<registered plugin alias>. (The plugin@name attribute is unused in this case and can conventionally use the same plugin name or alias stem as the plugin@filename attribute).

I updated the SystemLoader::InstantiateSystemPlugin method to check if the plugin filename conforms to the above pattern and attempt to instantiate the plugin from the static registry in this case. Loading fails if the plugin either cannot be found in the static registry or it does not implement the System interface.

I also added an integration test in test/integration/load_system_static_registry.cc to verify this feature (runs both in cmake and bazel build). This test uses the TestModelSystem test System class that I registered with the static plugin registry in test/plugins/TestStaticModelSystem.cc. Note that the TestStaticModelSystem static library needs to be linked with special flags into the test binary target in cmake to ensure that the plugin class symbols are not stripped out by the linker (similar to this test target in gz plugin).

Test it

bazel:

$ bazel test test:INTEGRATION_load_system_static_registry

cmake:

$ colcon test --merge-install --event-handlers console_direct+ --packages-select gz-sim9 --ctest-args -R INTEGRATION_load*

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, just minor style suggestions

[iche033]

I think gz-transport is not needed here

[shameekganguly]

I believe it is needed since the plugin implementation in the header file test/plugins/TestModelSystem.hh exposes a gz transport service.