Datagram Transport Integration

- Implement datagram transport adaptor, which wraps datagram transport in DtlsTransportInternal. Datagram adaptor owns both ICE and Datagram Transports.
- Implement setup of datagram transport based on RTCConfiguration flag use_datagram_transport. This is very similar to MediaTransport setup with the exception that we create DTLS datagram adaptor.
- Propagate maximum datagram size to video encoder via MediaTransportConfig.

TODO: Currently this CL can only be tested in downstream projects. Once we add fake datagram transport, we will be able to implement unit tests similar to loopback media transport.

Bug: webrtc:9719
Change-Id: I4fa4a5725598dfee5da4f0f374269a7e289d48ed
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/138100
Commit-Queue: Anton Sukhanov <sukhanov@webrtc.org>
Reviewed-by: Bjorn Mellem <mellem@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28047}

media/base/rtp_data_engine_unittest.cc

@@ -74,8 +74,7 @@ class RtpDataMediaChannelTest : public ::testing::Test {
     cricket::MediaConfig config;
     cricket::RtpDataMediaChannel* channel =
         static_cast<cricket::RtpDataMediaChannel*>(dme->CreateChannel(config));
-    channel->SetInterface(iface_.get(), webrtc::MediaTransportConfig(
-                                            /*media_transport=*/nullptr));
+    channel->SetInterface(iface_.get(), webrtc::MediaTransportConfig());
     channel->SignalDataReceived.connect(receiver_.get(),
                                         &FakeDataReceiver::OnDataReceived);
     return channel;