[pjrt] Add PjRtFuture<void> specialization for stateless futures
[pjrt] Add PjRtFuture<void> specialization for stateless futures

PjRtFuture<void> (or PjRtFuture<>) is an implicit wrapper for absl::Status that will become available at some point in the future and will signal completion of an async event.

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/openxla/xla/pull/11205 from ROCm:ci_link_break_20240404 8ee9b5e90d4e1a3ca7734bbe1ef73140e4d879dd
