Add an option to make bucket notifications synchronous (#17406)

With the current asynchronous behaviour in sending notification events
to the targets, we can't provide guaranteed delivery as the systems
might go for restarts.

For such event-driven use-cases, we can provide an option to enable
synchronous events where the APIs wait until the event is successfully
sent or persisted.

This commit adds 'MINIO_API_SYNC_EVENTS' env which when set to 'on'
will enable sending/persisting events to targets synchronously.

internal/event/targetlist_test.go

@@ -249,7 +249,7 @@ func TestTargetListSend(t *testing.T) {
 	for i, testCase := range testCases {
 		testCase.targetList.Send(Event{}, map[TargetID]struct{}{
 			testCase.targetID: {},
-		}, resCh)
+		}, resCh, false)
 		res := <-resCh
 		expectErr := (res.Err != nil)