Attach API support

[myllynen]

Parfait should provide a helper utility to allow flexibly attaching Parfait agent to an already running VM. This is possible with the Attach API which is an integral part of JVM since Java 9 (this would work also on Java 8 as long as the tools.jar (which is not needed with Java 9 and later) is available).

This approach would allow gathering metrics with Parfait from an earlier started VM without the need for an additional JVM process once the helper has loaded the agent.