Add Test Coverage Tooling

[amondel2]

When updating this plugin to Grails 7, I realized how dependent we are on running tests when making changes, yet we had little insight into how well the tests actually covered the code. During discussions, several possible tools were considered, and although it’s certainly not perfect, we are leaning toward using JaCoCo after weighing the pros and cons. Rather than implementing this across all projects at once, I decided to start with this plugin and expand outward if it proves successful and provides a solid template for future use.