Add CMake plugin directories to Simulate

[DebadityaHait]

Summary

• add SIMULATE_ADDITIONAL_PLUGIN_DIRS to authorize extra plugin directories at configure time
• keep scanning the executable-relative mujoco_plugin directory
• add regression coverage for default and configured plugin directories

Fixes #3357

Testing

• cmake --build build-validation --config Release --target simulate_plugin_test --parallel 4
• ctest --test-dir build-validation -C Release -R SimulatePluginTest --output-on-failure
• cmake --build build-validation --config Release --target simulate --parallel 4