[pjrt] NFC: Add use-declared move constructor/assignment to PjRtFuture
[pjrt] NFC: Add use-declared move constructor/assignment to PjRtFuture

User-declared move constructor/assignment implicitly deletes copy constructor/assignment. It is deleted today anyway because AsyncValueRef is not copyable, but if that will change we don't want PjRtFuture to become copyable because OnReady API assumes that value moved-out to the callback.
