Integrated support for Gazebo clock and use_sim_time

[evan-palmer]

Checklist

• I have performed a thorough review of my code
• I have sufficiently commented my code
• The implementation follows the project style conventions
• All project unit tests are passing
• If relevant, documentation has been provided or updated to discuss the changes made
• System integration tests were performed successfully

Changes Made

This PR introduces support for using the Gazebo clock as the system clock. This makes it possible for folks to now use either the Gazebo clock or the system clock according to their needs. Furthermore, this PR improves the modularity of the launch files to make it easier to integrate support for new models and modify the configurations of existing models. I also removed Foxglove "support". This project doesn't have any particular configuration files for visualization, so this isn't really a feature. Additionally, it requires users to install an external package that the may not need or want.

Associated Issues

• Fixes [FEATURE]: Improve launch description modularity #47
• Fixes [FEATURE]: Implement use_sim_time argument #48

Testing

Regression testing was performed using SITL.