Clarify whether the artifact sharing plugin is thread safe

When executing under Maven 3.x with parallel build (-T 1C), a warning appears saying that bamboo-artifact-sharing-maven-plugin is unknown to be threadsafe.

To avoid potential errors, the @threadSafe annotation has to be added to the plugin, with the value being true/false as appropriate. That way, customers will know for sure whether the plugin is thread safe or not.