Add design & decision log for Transfer Manager Progress Listeners

docs/design/services/s3/transfermanager/listener/DecisionLog.md

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+# Decision Log for v2 Transfer Manager Progress Listeners
+
+## Log Entry Template
+
+**Source:** (Meeting/aside/pair programming discussion/daily standup) to (discuss/implement) X
+
+**Attendees:** (names)
+
+**Closed Decisions:**
+
+1. Question? Decision. Justification.
+
+**Open Decisions:**
+
+1. (Old/Reopened/New) Question?
+
+## 9/21/21
+
+**Source:** Meeting to discuss implementing progress listeners for [TransferManager v2](https://github.com/aws/aws-sdk-java-v2/tree/master/docs/design/services/s3/transfermanager)
+
+**Attendees:** Anna-Karin, Bennett, Dongie, John, Matt, Zoe
+
+**Closed Decisions:**
+
+1. **Should ExecutionListener be implemented (under the hood) as an ExecutionInterceptor?**
+    1. We believe yes, but that would require ExecutionListener's API to inherently be a sub-set of ExecutionInterceptor,
+       which may be a limitation. Where needed, we can consider *adding* methods to the Interceptor interface to support
+       the Listener interface. Furthermore, we can consider migrating some parts of the metric implementation to be
+       Listener-based.
+1. **Should `TransferProgress` be mutable or immutable?**
+    1. We want to strive for the consistent v2 design tenant of immutability, but `TransferProgress` is often used in a
+    poll-based way to check the status of a transfer. We agreed that `TransferProgress` should be immutable, but we
+    could implement this by taking a *snapshot* of an immutable object, i.e., `TransferProgressSnapshot`.
+1. **What level of CRT support/integration is needed?**
+    1. We believe that the CRT's [ResponseDataConsumer](https://github.com/awslabs/aws-crt-java/blob/main/src/main/java/s3NativeClient/com/amazonaws/s3/ResponseDataConsumer.java#L22)
+    may be sufficient to implement the proposed TransferListener interface. Therefore we believe we can defer the
+    implementation of ExecutionListeners until required.
+
+**Open Decisions:**
+
+1. **Do we need the functionality proposed in `ExecutionMode`?**
+    1. Unknown. We may wish to reserve it only for async clients (which already have dedicated thread pools), and it may
+       be cleaner to allow users to define their own offloading semantics. We also would need consistent with allowing
+       users to provide async future completion, where Runnable::run would be analogous to the proposed "INLINE".
+       Deferring this idea until we dive deeper.