The handshakes support for virtual threads is needed to simplify the JVMTI implementation for virtual threads. There is a significant duplication in the JVMTI code to differentiate code intended to support platform, virtual threads or both. The handshakes have to be unified, so it should enough to define just one handshake for both platform and virtual thread. This implementation will be based on the JvmtiVTMSTransitionDisabler class.

The interface includes two new classes:
  - JvmtiHandshake, JvmtiUnifiedHandshakeClosure

The first JVMTI functions are picked to be converted to use JvmtiHandshake are:
 - GetStackTrace, GetFrameCount, GetFrameLocation, NotifyFramePop