Add an option to the Maven plugin to specify which Java agent to use

Currently, TraceMOP's Maven plugin uses the pre-built agents `track-no-stats-agent.jar` (for trace collection) and `no-track-no-stats-agent.jar` (no trace collection).

However, users may sometimes want to use a custom built agent. So we should add an option to the Maven plugin that allows users to specify the path to the agent they want to use.
