Add experimental plugin support

This commit focuses on adding support for Go plugins for further testing experimentation without breaking any existing RunAs/Federation functionality.

• Moved all support structures that are used for script environment as well as now plugin support for a secondary repo (gitlab.com/ecp-ci/jacamar-plugins) to address previously identified issues with project pkg + CGO.
• Expanded Pavilion testing to build and test a basic validation plugin.
• Removed unused go tests
• Federation username now only provided via FEDEREATED_USERNAME environment varaible when using RunAs validation script.

In addtion to these changes several small code quality enhancements have been made to the authuser/validation package.