Initial implementation of async execution.

* Only implements async execution and asynd d2h.
* Need to come up with a better way to bridge async completion signals to EventInstance::OnReady callback.
* There is room here for more of a scheduler that has some knowledge of the transfer and execution timeline. Will be needed when joining h2d transfer timelines.

Author: stellaraccident

pjrt-plugin/iree/integrations/pjrt/common/BUILD

@@ -26,6 +26,7 @@ iree_pjrt_cc_library(
     deps = [
         ":compiler",
         ":debugging",
+        "@iree_core//runtime/src/iree/base:tracing",
         "@iree_core//runtime/src/iree/hal",
         "@iree_core//runtime/src/iree/modules/hal",
         "@iree_core//runtime/src/iree/vm",