Improve process instrumentation for Java 9

**Problem description**

This is an improvement on elastic/apm-agent-java#903 (external process execution instrumentation).

Starting from Java 9, there is a new API available for process execution `ProcessHandle`, which is not covered by #903.
As a result, any usage of this API will not result in spans be created.

**Proposed solution**

Instrument `Process.toHandle()` to wrap returned `ProcessHandle` instance.

Wrapped `ProcessHandle` should end current process span on those methods
- `ProcessHandle.destroy()`
- `ProcessHandle.destroyForcibly()`
- `ProcessHandle.onExit()` > returns a `CompletableFuture` which should be wrapped and terminate span

This would also allow to use `ProcessHandle.Info` and capture program arguments, PID or other attributes.

Alternatively, we could also use `CompletableFuture.thenApply(...)` to chain a task that will terminate the process span.