dcsctp: Add API to indicate packet send status

Before this change, there was no way for a client to indicate to the
dcSCTP library if a packet that was supposed to be sent, was actually
sent. It was assumed that it always was.

To handle temporary failures better, such as retrying to send packets
that failed to be sent when the send buffer was full, this information
is propagated to the library.

Note that this change only covers the API and adaptations to clients.
The actual implementation to make use of this information is done as a
follow-up change.

Bug: webrtc:12943
Change-Id: I8f9c62e17f1de1566fa6b0f13a57a3db9f4e7684
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/228563
Commit-Queue: Victor Boivie <boivie@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34767}

net/dcsctp/socket/mock_context.h

@@ -42,7 +42,7 @@ class MockContext : public Context {
     ON_CALL(*this, callbacks).WillByDefault(testing::ReturnRef(callbacks_));
     ON_CALL(*this, current_rto).WillByDefault(testing::Return(DurationMs(123)));
     ON_CALL(*this, Send).WillByDefault([this](SctpPacket::Builder& builder) {
-      callbacks_.SendPacket(builder.Build());
+      callbacks_.SendPacketWithStatus(builder.Build());
     });
   }